RECOMBINANT MEGALOMICIN BIOSYNTHETIC GENES AND USES THEREOF

Abstract: Recombinant nucleic acid, <i>e.g</i> DNA compounds that encode all or a portion of the megalomicin polyketide synthase and modification enzymes are used to express recombinant polyketide synthase genes in host cells for the production of megalomicin, megalomicin derivatives, and other polyketides that are useful as antibiotics, motilides, and antiparasitics.

Description: Title Recombinant Megalomicin Biosynthetic Genes And Uses Thereof Cross-Reference to Priority Application This application claims priority to provisional U.S. patent application Serial No. 60/158,305, filed 8 October 1999, and provisional U.S. patent application Serial No. 60/190,024, filed 17 March 2000 under 35 U.S.C. § 1 19(e). The content of the above referenced applications is incorporated herein by reference in its entirety. Field of the Invention The present invention provides recombinant methods and materials for producing polyketides by recombinant DNA technology. The invention relates to the fields of agriculture, animal husbandry, chemistry, medicinal chemistry, medicine, molecular biology, pharmacology, and veterinary technology. Background of the Invention Polyketides represent a large family of diverse compounds synthesized from 2-carbon units through a series of condensations and subsequent modifications. Polyketides occur in many types of organisms, including fungi and mycelial bacteria, in particular, the actinomycetes. There are a wide variety of polyketide structures, and the class of polyketides encompasses numerous compounds with diverse activities. Erythromycin, FK-506, FK-520, megalomicin. narbomycin, oleandomycin, picromycin, rapamycin, spinocyn, and tylosin are examples of such compounds. Given the difficulty in producing polyketide compounds by traditional chemical methodology, and the typically low production of polyketides in wild-type cells, there has been considerable interest in finding improved or alternate means to produce polyketide compounds. See PCT publication Nos. WO 93/13663; WO 95/08548; WO 96/40968; WO 97/02358; and WO 98/27203; United States Patent Nos. 4,874,748; 5,063,155; 5,098,837; 5,149,639; 5,672,491 ; and 5,712,146; Fu et al, 1994, Biochemistry 33: 9321- 9326; McDaniel et al., 1993, Science 262: 1546-1550; and Rohr, 1995, Angew. Chem. Int. Ed. Engl. 34(8): 881-888, each of which is incorporated herein by reference. Polyketides are synthesized in nature by polyketide synthase (PKS) enzymes. These enzymes, which are complexes of multiple large proteins, are similar to the synthases that catalyze condensation of 2-carbon units in the biosynthesis of fatty acids. PKS enzymes are encoded by PKS genes that usually consist of three or more open reading frames (ORFs). Two major types of PKS enzymes are known; these differ in their composition and mode of synthesis. These two major types of PKS enzymes are commonly referred to as Type I or "modular" and Type II "iterative" PKS enzymes. Modular PKSs are responsible for producing a large number of 12-, 14-, and 16-membered macrolide antibiotics including erythromycin, megalomicin, methymycin, narbomycin, oleandomycin, picromycin, and tylosin. Each ORF of a modular PKS can comprise one, two, or more "modules" of ketosynthase activity, each module of which consists of at least two (if a loading module) and more typically three (for the simplest extender module) or more enzymatic activities or "domains." These large multifunctional enzymes (>300,000 kDa) catalyze the biosynthesis of polyketide macrolactones through multistep pathways involving decarboxylative condensations between acyl thioesters followed by cycles of varying β-carbon processing activities (see O'Hagan, D. The polyketide metabolites; E. Horwood: New York, 1991, incorporated herein by reference). During the past half decade, the study of modular PKS function and specificity has been greatly facilitated by the plasmid-based Streptomyces coelicolor expression system developed with the 6-deoxyerythronolide B (6-dEB) synthase (DEBS) genes (see Kao et al, 1994, Science, 265: 509-512, McDaniel et al., 1993, Science 262: 1546-1557, and U.S. Patent Nos. 5,672,491 and 5,712,146, each of which is incorporated herein by reference). The advantages to this plasmid-based genetic system for DEBS are that it overcomes the tedious and limited techniques for manipulating the natural DEBS host organism, Saccharopolyspora erythraea, allows more facile construction of recombinant PKSs, and reduces the complexity of PKS analysis by providing a "clean" host background. This system also expedited construction of the first combinatorial modular polyketide library in Streptomyces (see PCT publication No. WO 98/49315, incorporated herein by reference). The ability to control aspects of polyketide biosynthesis, such as monomer selection and degree of β-carbon processing, by genetic manipulation of PKSs has stimulated great interest in the combinatorial engineering of novel antibiotics (see Hutchinson, 1998, Curr. Opin. Microbiol. 1: 319-329; Carreras and Santi, 1998, Curr. Opin. Biotech. 9: 403-411 ; and U.S. Patent Nos. 5,712,146 and 5,672,491, each of which is incorporated herein by reference). This interest has resulted in the cloning, analysis, and manipulation by recombinant DNA technology of genes that encode PKS enzymes. The resulting technology allows one to manipulate a known PKS gene cluster either to produce the polyketide synthesized by that PKS at higher levels than occur in nature or in hosts that otherwise do not produce the polyketide. The technology also allows one to produce molecules that are structurally related to, but distinct from, the polyketides produced from known PKS gene clusters. Megalomicin is a macrolide antibiotic produced by Micromonospora megalomicea, a member of the Actinomycetales family of soil bacteria that produces many types of biologically active compounds. Megalomicin is a glycoside of erythromycin A, a widely used antibacterial drug with little or no antimalarial activity. Megalomicin has antibacterial properties similar to those of erythromycin, and in 1998, it was discovered also to have potent antiparasitic activity and low toxicity. The antiparasitic activity may be related to the effect megalomicin has on protein trafficking in eukaryotes, where it appears to inhibit vesicular transport between the medial and trans-Golgi, resulting in under- sialylation of proteins. Hence, megalomicin offers an exciting opportunity to develop a new class of antiparasitic drugs with a different mechanism of action than the drugs currently in use and, therefore, possibly active against drug-resistant forms of Plasmodium falciparum. The number and diversity of megalomicin derivatives have been limited due to the inability to manipulate the PKS genes, which have not previously been available in recombinant form. Genetic systems that allow rapid engineering of the megalomicin biosynthetic genes would be valuable for creating novel compounds for pharmaceutical, agricultural, and veterinary applications. The production of such compounds could be more readily accomplished if the heterologous expression of the megalomicin biosynthetic genes in Streptomyces coelicolor and S. lividans and other host cells were possible. The present invention meets these and other needs. Summary of the Invention The present invention provides recombinant methods and materials for expressing PKS enzymes and polyketide modification enzymes derived in whole and in part from the megalomicin biosynthetic genes in recombinant host cells. The invention also provides the polyketides produced by such PKS enzymes. The invention provides in recombinant form all of the genes for the proteins that constitute the complete PKS that ultimately results, in Micromonospora megalomicea, in the production of megalomicin. Thus, in one embodiment, the invention is directed to recombinant materials comprising nucleic acids with nucleotide sequences encoding at least one domain, module, or protein encoded by a megalomicin PKS gene. In one preferred embodiment of the invention, the DNA compounds of the invention comprise a coding sequence for at least one and preferably two or more of the domains of the loading module and extender modules 1 through 6, inclusive, of the megalomicin PKS. In one embodiment, the invention provides a recombinant expression vector that comprises a heterologous promoter positioned to drive expression of one or more of the megalomicin biosynthetic genes. In a preferred embodiment, the promoter is derived from another PKS gene. In a related embodiment, the invention provides recombinant host cells comprising one or more expression vectors that produce(s) megalomicin or a megalomicin derivative or precursor. In a preferred embodiment, the host cell is Streptomyces lividans or S. coelicolor. In another embodiment, the invention provides a recombinant expression vector that comprises a promoter positioned to drive expression of a hybrid PKS comprising all or part of the megalomicin PKS and at least a part of a second PKS. In a related embodiment, the invention provides recombinant host cells comprising the vector that produces the hybrid PKS and its corresponding polyketide. In a preferred embodiment, the host cell is Streptomyces lividans or S. coelicolor. In a related embodiment, the invention provides recombinant materials for the production of libraries of polyketides wherein the polyketide members of the library are synthesized by hybrid PKS enzymes of the invention. The resulting polyketides can be further modified to convert them to other useful compounds, such as antibiotics, motilides, and antiparasitics, typically through hydroxylation and/or glycosylation. Modified macrolides provided by the invention that are useful intermediates in the preparation of antiparasitics are of particular benefit. In another related embodiment, the invention provides a method to prepare a nucleic acid that encodes a modified PKS, which method comprises using the megalomicin PKS encoding sequence as a scaffold and modifying the portions of the nucleotide sequence that encode enzymatic activities, either by mutagenesis, inactivation, deletion, insertion, or replacement. The thus modified megalomicin PKS encoding nucleotide sequence can then be expressed in a suitable host cell and the cell employed to produce a polyketide different from that produced by the megalomicin PKS. In addition, portions of the megalomicin PKS coding sequence can be inserted into other PKS coding sequences to modify the products thereof. In another related embodiment, the invention is directed to a multiplicity of cell colonies, constituting a library of colonies, wherein each colony of the library contains an expression vector for the production of a modular PKS derived in whole or in part from the megalomicin PKS. Thus, at least a portion of the modular PKS is identical to that found in the PKS that produces megalomicin and is identifiable as such. The derived portion can be prepared synthetically or directly from DNA derived from organisms that produce megalomicin. In addition, the invention provides methods to screen the resulting polyketide and antibiotic libraries. The invention also provides novel polyketides, motilides, antibiotics, antiparasitics and other useful compounds derived therefrom. The compounds of the invention can also be used in the manufacture of another compound. In a preferred embodiment, the compounds of the invention are formulated in a mixture or solution for administration to an animal or human. In a specific embodiment, the invention provides an isolated nucleic acid fragment comprising a nucleotide sequence encoding a domain of megalomicin polyketide synthase (PKS) or a megalomicin modification enzyme. The isolated nucleic acid fragment can be a DNA or a RNA. Preferably, the isolated nucleic acid fragment is a recombinant DNA compound. The isolated nucleic acid fragment can comprise a single, multiple or all the open reading frame(s) (ORF) of the megalomicin PKS or a megalomicin modification enzyme. Exemplary ORFs of megalomicin PKS include the ORFs of the megAI, megAII and megAIII genes. The isolated nucleic acid fragment can also encode a single, multiple, or all of the domains of the megalomicin PKS. Exemplary domains of the megalomicin PKS include a TE domain, a KS domain, an AT domain, an ACP domain, a KR domain, a DH domain and an ER domain. In a preferred embodiment, the nucleic acid fragment encodes a module of the megalomicin PKS. In another preferred embodiment, the nucleic acid fragment encodes the loading module, a thioesterase domain, and all six extender modules of the megalomicin PKS. Megalomicin modification enzymes include those enzymes involved in the conversion of 6-dEB into a megalomicin such as the enzymes encoded by the megF, meg BV, megCIJI, megK, megDI and megG (renamed megϊ) genes. Megalomicin modification enzymes also include those enzymes involved in the biosynthesis of mycarose, megosamine or desosamine, which are used as biosynthetic intermediates in the biosynthesis of various megalomicin species and other related polyketides. The enzymes that are involved in biosynthesis of mycarose, megosamine or desosamine are described in Figures 5 and 10. In a preferred embodiment, the invention provides an isolated nucleic acid fragment which hybridizes to a nucleic acid having a nucleotide sequence set forth in the SEQ. ID NOT, under low, medium or high stringency. More preferably, the nucleic acid fragment comprises, consists or consists essentially of a nucleic acid having a nucleotide sequence set forth in the SEQ. ID NO: 1. In another specific embodiment, the invention provides a substantially purified polypeptide, which is encoded by a nucleic acid fragment comprising a nucleotide sequence encoding a domain of megalomicin polyketide synthase (PKS) or a megalomicin modification enzyme. The polypeptide can comprise a single domain, multiple domains or a full-length megalomicin PKS or megalomicin modification enzyme. Functional fragments, analogs or derivatives of the megalomicin PKS or megalomicin modification enzyme polypeptides are also provided. Preferably, such fragments, analogs or derivatives can be recognized by an antibody raised against a megalomicin PKS or megalomicin modification enzyme. Also preferably, such fragments, analogs or derivatives comprise an amino acid sequence that has at least 60% identity, more preferably at least 90% identity, to their wild type counterparts. In still another specific embodiment, the invention provides an antibody, or a fragment or derivative thereof, which immuno-specifically binds to a domain of megalomicin polyketide synthase (PKS) or a megalomicin modification enzyme. The antibody can be a monoclonal or polyclonal antibody or an antibody fragment. Preferably, the antibody is a monoclonal antibody. In yet another specific embodiment, the invention provides a recombinant DNA expression vector comprising the recombinant DNA compound encoding at least a domain of the megalomicin PKS or a megalomicin modification enzyme, wherein said domain is operably linked to a promoter. Preferably, the recombinant DNA expression vector further comprises an origin of replication or a segment of DNA that enables chromosomal integration. In yet another specific embodiment, the invention provides a recombinant host cell comprising the above-described recombinant DNA expression vector encoding at least a domain of megalomicin PKS or the megalomicin modification enzyme. The recombinant host cells can be any suitable host cells including animal, mammalian, plant, fungal, yeast, and bacterial cells. Preferably, the recombinant host cells are Streptomyces cells, such as Streptomyces lividans and S. coelicolor cells, or ccharopolyspora cells, such as Saccharopolyspora erythraea cells. Also preferably, the recombinant host cells do not produce megalomicin in their untransformed, non-recombinant state. When the recombinant host cell contains nucleic acid encoding more than one megalomicin PKS or megalomicin modification enzyme, or domains thereof, such nucleic acid material can be located at a single genetic locus, e.g., on a single plasmid or at a single chromosomal locus, or at different genetic loci, e.g , on separate plasmids and/or chromosomal loci. In one example, the invention provides a recombinant host cell, which comprises at least two separate autonomously replicating recombinant DNA expression vectors, and each of said vectors comprises a recombinant DNA compound encoding a megalomicin PKS domain or a megalomicin modification enzyme operably linked to a promoter. In another example, the invention provides a recombinant host cell, which comprises at least one autonomously replicating recombinant DNA expression vector and at least one modified chromosome, each of said vector(s) and each of said modified chromosome comprises a recombinant DNA compound encoding a megalomicin PKS domain or a megalomicin modification enzyme operably linked to a promoter. Preferably, the autonomously replicating recombinant DNA expression vector and/or the modified chromosome further comprises distinct selectable markers. In a preferred embodiment, the cell comprises three different vectors, one of which is integrated into the chromosome and two of which are autonomously replicating, and each of the vectors comprises a meg PKS gene. Optionally, one or more of the meg PKS genes contains one or more domain alterations, such as a deletion or substitution of a meg PKS domain with a domain from another PKS. In yet another specific embodiment, the invention provides a hybrid PKS, which is produced from a recombinant gene that comprises at least a portion of a megalomicin PKS gene and at least a portion of a second PKS gene for a polyketide other than megalomicin. For example, and without limitation, the second PKS gene can be a narbonolide PKS gene, an oleandolide PKS gene, or a rapamycin PKS gene. In one embodiment, the hybrid PKS is composed of a loading module and six extender modules, wherein at least one domain of any one of extender modules 1 through 6, inclusive, is a domain of an extender module of megalomicin PKS. In another preferred embodiment, the hybrid PKS comprises a megalomicin PKS that has a non-functional KS domain in module 1. In yet another specific embodiment, the invention provides a method of producing a polyketide, which method comprises growing the recombinant host cell comprising a recombinant DNA expression vector encoding at least a domain of the megalomicin PKS or a megalomicin modification enzyme under conditions whereby the megalomicin PKS domain or the megalomicin modification enzyme comprised by the recombinant expression vector is produced and the polyketide is synthesized by the cell, and recovering the synthesized polyketide. Preferably, the recombinant host cell comprises a recombinant expression vector that encodes at least a portion of a megAI, megAII, or megAIII gene. These and other embodiments of the invention are described in more detail in the following description, the examples, and claims set forth below. Brief Description of the Figures Figure 1 shows restriction site and function maps of the insert DNA in cosmids pKOS079-138B, pKOS079-93D, pKOS079-93A, and pKOS079-124B of the invention. Various restriction sites (Xhol, Bglll, Nsil) are also shown. The location of the megalomicin biosynthetic genes is shown below the solid lines indicating the cosmid inserts. The genes are shown as arrows pointing in the direction of transcription. The approximate size (in kilobase (kb) pairs) of the gene cluster is indicated in 5000 bp (i.e., 5K, 10K, and the like.) increments on a solid bar beneath the arrows indicating the genes. Figure 2 shows a more detailed map of the megalomicin biosynthetic gene cluster. The various open reading frames are shown as arrows pointing in the direction of transcription. A line indicates the size in base pairs (in 1000 bp increments) of the gene cluster. The various domains of the megalomicin PKS are also shown. Other genes of the megalomicin biosynthetic gene cluster not shown in this Figure are located in the insert DNA of cosmids pKOSOl 38B and pKOS0124B. Figure 3 shows the structures of the megalomicins, azithromycin and erythromycin A. Figure 4 shows the modules and domains of DEBS and the megalomicin PKS. Figure 5 shows the compounds and reactions in the erythromycin biosynthetic pathway and also for megalomicin biosynthesis. Genes that produce the various enzymes that catalyze each of the steps in the biosynthetic pathway are indicated. Figure 6 shows the biosynthetic pathway for the formation of desosamine, rhodosamine, and mycarose, as well as the genes that produce the various enzymes that catalyze each of the steps in the biosynthetic pathway. Figure 7 depicts nucleotide and amino acid sequence of Micromonospora megalomicea megalomicin biosynthetic genes (GenBank Accession No. AF263245, incorporated herein by reference). Figure 8 depicts the biosynthesis of the eryfhromycins and megalomicins and the enzymes that mediate the biosynthesis of each. Figure 9 depicts the cloned megalomicin biosynthetic gene cluster and certain cosmids of the invention that comprise portions of the cluster. Figure 10 depicts the biosynthesis of megosamine, mycarose, and desosamine. Detailed Description of the Invention The present invention provides useful compounds and methods for producing polyketides in recombinant host cells. As used herein, the term recombinant refers to a compound or composition produced by human intervention. The invention provides recombinant DNA compounds encoding all or a portion of the megalomicin biosynthetic genes. The invention provides recombinant expression vectors useful in producing the megalomicin PKS and hybrid PKSs composed of a portion of the megalomicin PKS in recombinant host cells. The invention also provides the polyketides produced by the recombinant PKS and polyketide modification enzymes. To appreciate the many and diverse benefits and applications of the invention, the description of the invention below is organized as follows. In Section I, common definitions used throughout this application are provided. In Section II, structural and functional characteristics of megalomicin are described. In Section III, the recombinant megalomicin biosynthetic genes and other recombinant nucleic acids provided by the invention are described. In Section IV, polypeptides and proteins encoded by the megalomicin biosynthetic genes and antibodies that specifically bind to such polypeptides and proteins provided by the invention are described. In Section V, methods for heterologous expression of the megalomicin biosynthetic genes provided by the invention are described. In Section VI, the hybrid PKS genes provided by the invention are described. In Section VII, host cells containing multiple megalomicin biosynthetic genes and nucleic acid fragments on separate express vectors provided by the invention are described. In Section VIII, the polyketide compounds provided by the invention and pharmaceutical compositions of those compounds are described. The detailed description is followed by working examples illustrating the invention. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this invention belongs. All patents, applications, published applications and other publications and sequences from GenBank and other data bases referred to herein are incorporated by reference in their entirety. Section I. Definitions As used herein, domain refers to a portion of a molecule, e.g., proteins or nucleic acids, that is structurally and/or functionally distinct from another portion of the molecule. As used herein, antibody includes antibody fragments, such as Fab fragments, which are composed of a light chain and the variable region of a heavy chain. As used herein, biological activity refers to the in vivo activities of a compound or physiological responses that result upon in vivo administration of a compound, composition or other mixture. Biological activity, thus, encompasses therapeutic effects and pharmaceutical activity of such compounds, compositions and mixtures. Biological activities may be observed in in vitro systems designed to test or use such activities. As used herein, a combination refers to any association between two or among more items. As used herein, a composition refers to any mixture. It may be a solution, a suspension, liquid, powder, a paste, aqueous, non-aqueous or any combination thereof As used herein, derivative or analog of a molecule refers to a portion derived from or a modified version of the molecule. As used herein, operably linked, operatively linked or operationally associated refers to the functional relationship of DNA with regulatory and effector sequences of nucleotides, such as promoters, enhancers, transcriptional and translational stop sites, and other signal sequences. For example, operative linkage of D A to a promoter refers to the physical and functional relationship between the DNA and the promoter such that the transcription of such DNA is initiated from the promoter by an RNA polymerase that specifically recognizes, binds to and transcribes the DNA. To optimize expression and/or in vitro transcription, it may be helpful to remove, add or alter 5' untranslated portions of the clones to eliminate extra, potentially inappropriate alternative translation initiation (/. e. , start) codons or other sequences that may interfere with or reduce expression, either at the level of transcription or translation. Alternatively, consensus ribosome binding sites (see, e.g., Kozak, J Biol. Chem., 266: 19867- 19870 (1991)) can be inserted immediately 5' of the start codon and may enhance expression. The desirability of (or need for) such modification may be empirically determined. As used herein, pharmaceutically acceptable salts, esters or other derivatives of the conjugates include any salts, esters or derivatives that may be readily prepared by those of skill in this art using known methods for such derivatization and that produce compounds that may be administered to animals or humans without substantial toxic effects and that either are pharmaceutically active or are prodrugs. As used herein, a promoter region or promoter element refers to a segment of DNA or RNA that controls transcription of the DNA or RNA to which it is operatively linked. The promoter region includes specific sequences that are sufficient for RNA polymerase recognition, binding and transcription initiation. This portion of the promoter region is referred to as the promoter. In addition, the promoter region includes sequences that modulate this recognition, binding and transcription initiation activity of RNA polymerase. These sequences may be cis acting or may be responsive to trans acting factors. Promoters, depending upon the nature of the regulation, may be constitutive or regulated. As used herein: stringency of hybridization in determining percentage mismatch is as follows: (1) high stringency: 0.1 x SSPE, 0.1% SDS, 65°C; (2) medium stringency: 0.2 x SSPE, 0.1% SDS, 50°C; and (3) low stringency: 1.0 x SSPE, 0.1% SDS, 50°C. Equivalent stringencies may be achieved using alternative buffers, salts and temperatures. The term substantially identical or homologous or similar varies with the context as understood by those skilled in the relevant art and generally means at least 70%, preferably means at least 80%, more preferably at least 90%), and most preferably at least 95%> identity. As used herein, substantially identical to a product means sufficiently similar so that the property of interest is sufficiently unchanged so that the substantially identical product can be used in place of the product. As used herein, isolated means that a substance is either present in a preparation at a concentration higher than that substance is found in nature or in its naturally occurring state or that the substance is present in a preparation that contains other materials with which the substance is not associated with in nature. As an example of the latter, an isolated meg PKS protein includes a meg PKS protein expressed in a Streptomyces coelicolor or S. lividans host cell. As used herein, substantially pure means sufficiently homogeneous to appear free of readily detectable impurities as determined by standard methods of analysis, such as thin layer chromatography (TLC), gel electrophoresis and high performance liquid chromatography (HPLC), used by those of skill in the art to assess such purity, or sufficiently pure such that further purification would not detectably alter the physical and chemical properties, such as enzymatic and biological activities, of the substance. Methods for purification of the compounds to produce substantially chemically pure compounds are known to those of skill in the art. A substantially chemically pure compound may, however, be a mixture of stereoisomers or isomers. In such instances, further purification might increase the specific activity of the compound. As used herein, vector or plasmid refers to discrete elements that are used to introduce heterologous DNA into cells for either expression or replication thereof. Selection and use of such vehicles are well known within the skill of the artisan. An expression vector includes vectors capable of expressing DNAs that are operatively linked with regulatory sequences, such as promoter regions, that are capable of effecting expression of such DNA fragments. Thus, an expression vector refers to a recombinant DNA or RNA construct, such as a plasmid, a phage, recombinant virus or other vector that, upon introduction into an appropriate host cell, results in expression of the cloned DNA. Appropriate expression vectors are well known to those of skill in the art and include those that are replicable in eukaryotic cells and/or prokaryotic cells and those that remain episomal or those which integrate into the host cell genome. Section II. Megalomicins The megalomicins were discovered in 1969 at Schering Corp. as antibacterial agents produced by Micromonospora megalomicea (see Weinstein et al, 1969, J Antibiotics 22: 253-258, and U.S. Patent No. 3,632,750, both of which are incorporated herein by reference). Although the initial structural assignment was in error, a thorough reassessment of NMR data coupled with an X-ray crystal structure of a megalomicin A derivative (see Nakagawa and Omura, "Structure and Stereochemistry of Macrolides" in Macrolide Antibiotics (S. Omura, ed.), Academic Press, NY, 1984, incorporated herein by reference) established the structures shown in Figure 3. The megalomicins are 6-0- glycosides of erythromycin C with acetyl or propionyl groups esterified at the 3'" or 4'" hydroxyls of the mycarose sugar at the C-3-position. The C-6 sugar has been named "megosamine," although it had been identified 5 to 10 years earlier as L-rhodosamine or 7V-dimethyldaunosamine, deoxyamino sugars commonly present in the anthracycline antitumor drugs. The antibacterial potency, spectrum of activity, and toxicity (LD 50 acute, 7-7.5 g/kg s.c. or oral; subacute, >500 mg/kg) of the megalomicins is similar to that of erythromycin A. The megalomicins have two modes of biological activity. As antibacterials, they act like the erythromycins, which inhibit protein synthesis at the translocation step by selective binding to the bacterial 50S ribosomal RNA. They also affect protein trafficking in eukaryotic cells (see Bonay et al, 1996, J Biol. Chem. 277:3719-3726, incorporated herein by reference). Although the mechanism of action is not entirely clear, it appears to involve inhibition of vesicular transport between the medial and trans Golgi, resulting in under-sialylation of proteins. The megalomicins also strongly inhibit the ATP-dependent acidification of lysosomes in vivo (see Bonay et al, 1997, J. Cell. Sci. 110: 1839-1849, incorporated herein by reference) and cause an anomalous glycosylation of viral proteins, which may be responsible for their antiviral activity against herpes (Tox 50 , 70-100 μM; see Alarcon et al, 1984, Antivir. Res. 4:231-243, and Alarcon et al, 1988, FEBS Lett. 231 :207-21 1 , both of which are incorporated herein by reference). Strikingly, the megalomicins are potent antiparasitic agents, showing an IC 50 of 1 μg/ml in blocking intracellular replication of Plasmodium falciparum infected erythrocytes (see Bonay et al, 1998, Antimicrob. Agents Chemother. 42:2668-2673, incorporated herein by reference). The megalomicins are effective against Trypanosoma cruzi and T. brucei (IC50, 0.2-2 μg/ml) plus Leishmania donovani and L. major promastigotes (IC 50 , 3 and 8 μg/ml, respectively). Megalomicin is also active against the intracellular replicative, amastigote form of T. cruzi, completely preventing its replication in infected murine LLC/MK2 macrophages at a dose of 5 μg/ml. Importantly, the effective drug concentration is 500-fold less than the acute LD 50 in mammals, and there is no toxicity to BALB/c mice at doses (50 mg/kg) that are completely curative for T. brucei infections. Because the erythromycins do not have such activity, although azithromycin (Figure 3) has been reported to be an effective acute and prophylactic treatment for malaria caused by P. vivax and P. falciparum (see Taylor et al, 1999, Clin. Infect. Dis. 28:74-81, incorporated herein by reference), the antiparasitic action of the megalomicins is unique and probably related to the presence of the deoxyamino sugar megosamine at C-6 (Figure 3). Consequently, the megalomicins could be developed into potent antimalarial drugs with a high therapeutic index and be active against P. falciparum and other species that are resistant to currently used classes of antimalarials. They also could lead to potent antiparasitic agents against leishmaniasis, trypanosomiasis, and Chagas' disease. In view of the widespread use of the erythromycins and their good oral availability plus the low mammalian toxicity of macrolides in general, the megalomicins could be used prophylactically to combat malaria, and as fermentation products, the megalomicins should be relatively inexpensive to produce. . The megalomicins belong to the polyketide class of natural products whose members have diverse structural and pharmacological properties (see Monaghan and Tkacz, 1990, Annu. Rev. Microbiol 44: 271 , incorporated herein by reference). The megalomicins are assembled by polyketide synthases through successive condensations of activated coenzyme- A thioester monomers derived from small organic acids such as acetate, propionate, and butyrate. Active sites required for condensation include an acyltransferase (AT), acyl carrier protein (ACP), and beta-ketoacylsynthase (KS). Each condensation cycle results in a β- keto group that undergoes all, some, or none of a series of processing activities. Active sites that perform these reactions include a ketoreductase (KR), dehydratase (DH), and enoylreductase (ER). Thus, the absence of any beta-keto processing domain results in the presence of a ketone, a KR alone gives rise to a hydroxyl, a KR and DH result in an alkene, while a KR, DH, and ER combination leads to complete reduction to an alkane. After assembly of the polyketide chain, the molecule typically undergoes cyclization(s) and post-PKS modification (e.g. glycosylation, oxidation, acylation) to achieve the final active compound. Macrolides such as erythromycin and megalomicin are synthesized by modular PKSs (see Cane et al., 1998, Science 282: 63, incorporated herein by reference). For illustrative purposes, the PKS that produces the erythromycin polyketide (6-deoxyerythronolide B synthase or DEBS; see U.S. Patent No. 5,824,513, incorporated herein by reference) is shown in Figure 4. DEBS is the most characterized and extensively used modular PKS system. DEBS is particularly relevant to the present invention in that it synthesizes the same polyketide, 6-deoxyerythronolide B (6-dEB), synthesized by the megalomicin PKS. In modular PKS enzymes such as DEBS and the megalomicin PKS, the enzymatic steps for each round of condensation and reduction are encoded within a single "module" of the polypeptide (i.e., one distinct module for every condensation cycle). DEBS consists of a loading module and 6 extender modules and a chain terminating thioesterase (TE) domain within three extremely large polypeptides encoded by three open reading frames (ORFs, designated eryAI, eryAII, and eryAIII). Each of the three polypeptide subunits of DEBS (DEBSI, DEBSII, and DEBSIII) contains 2 extender modules, DEBSI additionally contains the loading module. Collectively, these proteins catalyze the condensation and appropriate reduction of 1 propionyl CoA starter unit and 6 methylmalonyl CoA extender units. Modules 1, 2, 5, and 6 contain KR domains; module 4 contains a complete set, KR/DH/ER, of reductive and dehydratase domains; and module 3 contains no functional reductive domain. Following the condensation and appropriate dehydration and reduction reactions, the enzyme bound intermediate is lactonized by the TE at the end of extender module 6 to form 6-dEB. More particularly, the loading module of DEBS consists of two domains, an acyl-transferase (AT) domain and an acyl carrier protein (ACP) domain. In other PKS enzymes, the loading module is not composed of an AT and an ACP but instead utilizes an inactivated KS, an AT, and an ACP. This inactivated KS is in most instances called KS , where the superscript letter is the abbreviation for the amino acid, glutamine, that is present instead of the active site cysteine required for activity. The AT domain of the loading module recognizes a particular acyl-CoA (propionyl for DEBS, which can also accept acetyl) and transfers it as a thiol ester to the ACP of the loading module. Concurrently, the AT on each of the extender modules recognizes a particular extender-CoA (methylmalonyl for DEBS) and transfers it to the ACP of that module to form a thioester. Once the PKS is primed with acyl- and malonyl-ACPs, the acyl group of the loading module migrates to form a thiol ester (trans-esterification) at the KS of the first extender module; at this stage, extender module 1 possesses an acyl-KS and a methylmalonyl ACP. The acyl group derived from the loading module is then covalently attached to the alpha-carbon of the malonyl group to form a carbon-carbon bond, driven by concomitant decarboxylation, and generating a new acyl-ACP that has a backbone two carbons longer than the loading unit (elongation or extension). The growing polyketide chain is transferred from the ACP to the KS of the next module, and the process continues. The polyketide chain, growing by two carbons each module, is sequentially passed as a covalently bound thiol ester from module to module, in an assembly line-like process. The carbon chain produced by this process alone would possess a ketone at every other carbon atom, producing a polyketone, from which the name polyketide arises. Commonly, however, the beta keto group of each two- carbon unit is modified just after it has been added to the growing polyketide chain but before it is transferred to the next module by either a KR, a KR plus a DH, or a KR, a DH, and an ER. As noted above, modules may contain additional enzymatic activities as well. Once a polyketide chain traverses the final extender module of a PKS, it encounters the releasing domain or thioesterase found at the carboxyl end of most PKSs. Here, the polyketide is cleaved from the enzyme and cyclyzed. The resulting polyketide can be modified further by tailoring or modification enzymes; these enzymes add carbohydrate groups or methyl groups, or make other modifications, i.e., oxidation or reduction, on the polyketide core molecule. For example, the final steps in conversion of 6-dEB to erythromycin A include the actions of a number of modification enzymes, such as: C-6 hydroxylation, attachment of mycarose and desosamine sugars, C-12 hydroxylation (which produces erythromycin C), and conversion of mycarose to cladinose via O- methylation, as shown in Figure 5. With this overview of PKS and post-PKS modification enzymes, one can better appreciate the recombinant megalomicin biosynthetic genes provided by the invention and their function, as described in the following Section. Section III: The Megalomicin Biosynthetic Genes and Nucleic Acid Fragments The megalomicin PKS was isolated and cloned by the following procedure. Genomic DNA was isolated from a megalomicin producing strain of Micromonospora megalomicea subsp. nigra (ATCC 27598), partially digested with a restriction enzyme, and cloned into a commercially available cosmid vector to produce a genomic library. This library was then probed with probe generated from the erythromycin biosynthetic genes as well as from cosmids identified as containing sequences homologous to erythromycin biosynthetic genes. This probing identified a set of cosmids, which were analyzed by DNA sequence analysis and restriction enzyme digestion, which revealed that the desired DNA had been isolated and that the entire PKS gene cluster was contained in overlapping segments on four of the cosmids identified. Figure 1 shows the cosmids, and the portions of the megalomicin biosynthetic gene cluster in the insert DNA of the cosmids. Figure 1 shows that the complete megalomicin biosynthetic gene cluster is contained within the insert DNA of cosmids pKOS079-138B, pKOS079-124B, pKOS079-93D, and pKOS079-93A. Each of these cosmids has been deposited with the American Type Culture Collection in accordance with the terms of the Budapest Treaty (cosmid pKOS079-138B is available under accession no. ATCC ; cosmid pKOS079-124B is available under accession no. ATCC ; cosmid pKOS079-93D is available under accession no. ATCC ; and cosmid pKOS079-93A is available under accession no. ATCC ). Various additional reagents of the invention can be isolated from these cosmids. DNA sequence analysis was also performed on the various subclones of the invention, as described herein. Further analysis of these cosmids and subclones prepared from the cosmids facilitated the identification of the location of various megalomicin biosynthetic genes, including the ORFs encoding the PKS, modules encoded by those ORFs, and coding sequences for megalomicin modification enzymes. The location of these genes and modules is shown on Figure 2. Those of skill in the art will recognize that, due to the degenerate nature of the genetic code, a variety of DNA compounds differing in their nucleotide sequences can be used to encode a given amino acid sequence of the invention. The native DNA sequence encoding the megalomicin PKS and other biosynthetic enzymes and other biosynthetic enzymes of Micromonospora megalomicea is shown herein merely to illustrate a preferred embodiment of the invention, and the invention includes DNA compounds of any sequence that encode the amino acid sequences of the polypeptides and proteins of the invention. In similar fashion, a polypeptide can typically tolerate one or more amino acid substitutions, deletions, and insertions in its amino acid sequence without loss or significant loss of a desired activity. The present invention includes such polypeptides with alternate amino acid sequences, and the amino acid sequences encoded by the DNA sequences shown herein merely illustrate preferred embodiments of the invention. The recombinant nucleic acids, proteins, and peptides of the invention are many and diverse. To facilitate an understanding of the invention and the diverse compounds and methods provided thereby, the following description of the various regions of the megalomicin PKS and the megalomicin modification enzymes and corresponding coding sequences is provided. To facilitate description of the invention, reference to a PKS, protein, module, or domain herein can also refer to DNA compounds comprising coding sequences therefor and vice versa. Also, unless otherwise indicated, reference to a heterologous PKS refers to a PKS or DNA compounds comprising coding sequences therefor from an organism other than Micromonospora megalomicea. In addition, reference to a PKS or its coding sequence includes reference to any portion thereof. Thus, the invention provides DNA molecules in isolated (i.e., not pure, but existing in a preparation in an abundance and/or concentration not found in nature) and purified (i.e., substantially free of contaminating materials or substantially free of materials with which the corresponding DNA would be found in nature) form. The DNA molecules of the invention comprise one or more sequences that encode one or more domains (or fragments of such domains) of one or more modules in one or more of the ORFs of the megalomicin PKS and sequences that encode megalomicin modification enzymes from the megalomicin biosynthetic gene cluster. Examples of PKS domains include the KS, AT, DH, KR, ER, ACP, and TE domains of at least one of the 6 extender modules and loading module of the three proteins encoded by the three ORFs of the megalomicin PKS gene cluster. Examples of megalomicin modification enzymes include those that synthesize the mycarose, desosamine, and megosamine moieties, those that transfer those sugar moieties to the polyketide 6-dEB, those that hydroxylate the polyketide at C-6 and C-12, and those that acylate the sugar moieties. In an especially preferred embodiment, the DNA molecule is a recombinant DNA expression vector or plasmid, as described in more detail in the following Section. Generally, such vectors can either replicate in the cytoplasm of the host cell or integrate into the chromosomal DNA of the host cell. In either case, the vector can be a stable vector (i.e., the vector remains present over many cell divisions, even if only with selective pressure) or a transient vector (i.e., the vector is gradually lost by host cells with increasing numbers of cell divisions). The megalomicin PKS gene cluster comprises three ORFs (megAI, megAII, and megAIII). Each ORF encodes two extender modules of the PKS; the first ORF also encodes the loading module. Each extender module is composed of at least a KS, an AT, and an ACP domain. The locations of the various encoding regions of these ORFs are shown in Figure 2 and described with reference to the sequence information below. The megalomicin PKS produces the polyketide known as 6- dEB, shown in Figure 4. In megalomicin-producing organisms, 6-dEB is converted to erythromycin C by a set of modi fication enzymes. Thus, 6-dEB is converted to erythronolide B by the megF gene product (a homolog of the eryF gene product), then to 3-alpha-mycarosyl-eryfhronolide B by the megBV gene product (a homolog of the eryBV gene product), then to erythromycin D by the megCIII gene product (a homolog of the eryCIII gene product, then to erythromycin C by the megK gene product (a homolog of the eryK gene product). In addition to these modification enzymes, such megalomicin-producing organisms also contain the modification enzymes necessary for the biosynthesis of the desosamine and mycarose moieties that are similarly utilized in erythromycin biosynthesis, as shown in Figure 5. Megalomicin A contains the complete erythromycin C structure, and its biosynthesis additionally involves the formation of L-megosamine (L-rhodosamine) and its attachment to the C-6 hydroxyl (Figures 3 and 5, inset), followed by acylation of the C-3"' and(or) C-4"' hydroxyls as the terminal steps. L-megosamine is the same as N-dimefhyl-L- daunosamine; the daunosamine genes have been characterized from Streptomyces peucetius (see Colombo and Hutchinson, J. Indus t. Microbiol. Biotechnol, in press; Otten et al, 1996, J Bacteriol 178:7316-7321 , and references cited therein). Some of the rhodosamine genes also have been cloned and partially characterized from another anthracycline producing Streptomyces sp. (see Torkkell et al., 1997, Mol. Gen. Genet. 25(5(2):203-209). Because the timing of the glycosylation with TDP-megosamine in relation to the addition of mycarose and desosamine to erythronolide B, plus the C-12 hydroxylation, is unknown, the pathway could involve a different order of glycosylation and C-12 hydroxylation steps than the one shown in Figure 5. Regardless, the megalomicin biosynthetic gene cluster contains the genes to make L-rhodosamine and attach it to the correct macrolide substrate. The biosynthetic pathways to make the glycosides desosamine, mycarose, and megosamine are shown in Figure 6. The present invention provides the genes for each biosynthetic pathway shown in this Figure, and these recombinant genetic pathways can be used alone or in any combination to confer the pathway to a heterologous host. The megalomicin PKS locus is similar to the eryA locus in size and organization. Most of the deoxysugar biosynthesis genes are homologs of the eryB mycarose and eryC desosamine biosynthesis and glycosyl attachment genes from Saccharopolyspora erythraea (see Summers et al, 1997, Microbiol. 143:3251- 3262; Haydock et al, 1991 , Mo/. Gen. Genet. 230: 120-128; Gaisser et al, 1997, Mol Gen Genet, 256:239-251 ; Gaisser et al, 1998, Mol Gen Genet. 257:78-88, incorporated herein by reference) or the picC homologs from the picromycin and narbomycin producer (see PCT patent publication No. 99/61599 and Xue et al., 1998, Proc. Nat. Acad. Sci. USA 95, 121 1 1-121 16, incorporated herein by reference). The TDP-megosamine biosynthesis genes are homologs of the dnm genes (see Figure 5) and the pikromycin N-dimethyltransferase gene or its homologs reported in a cluster of L-rhodosamine biosynthesis genes. The putative TDP-megosamine glycosyltransferase gene product (geneXm Figure 5) closely resembles the deduced products of the eryBV , eryCIII, dnmS, and pikromycin desVIl genes, even though it recognizes different substrates than the products of each of these genes. The following Table 1 shows the location of the genes in the Micromonospora megalomicea megalomicin biosynthetic pathway in the DNA sequence set forth in SEQ ID NOT (see also Figure 7; note some gene designations maybe different in Figure 7). Table 1. Megalomicin Biosynthetic Gene Cluster Micromonospora megalomicea subsp. nigra (ATCC27598) Location Description 1..2451 sequence from cosmid pKOS079-138B complement 1..144) megBVI (or megT), TDP-4-keto-6-deoxyglucose- 2,3-dehydratase 928..2061 megDVI, TDP-4-keto-6-deoxyglucose 3,4-isomerase 2072..3382 megDI, TDP-megosaminyl transferase (eryCIII homolog) 2452..40397 sequence of cosmid pKOS079-93D 3462..4634 megG(or megY), mycarosyl acyl transferase 4651..5775 megDII, deoxysugar transaminase (eryCI, DnrJ homolog) 5822..6595 megDIII, TDP-daunosaminyl-N,N- dimethyltransferase (eryCVI homolog) 6592..7197 megDIV, TDP-4-keto-6-deoxyglucose 3,5-epimerase (eryBVII, dnmll homolog) 7220..8206 megDV, TDP-hexose 4-ketoreductase (eryBIV, dnmV homolog) complement(8228..9220) megBIIΛ or megDV II, TDP-4-keto-L-6-deoxy- hexose 2,3-reductase complement(9226..10479) megBV, TDP-mycarosyl transferase complement(10483..1 1424) megBIV, TDP-hexose 4-ketoreductase 12181..22821 megAI 12181..13791 Loading Module (L) 12505..13470 AT-L 13576..13791 ACP-L 13849..18207 Extender Module 1 (1) 13849..15126 KS 1 15427..16476 ATI 17155..17694 KR1 17947..18207 ACPI 18268..22575 Extender Module 2 (2) 18268..19548 KS2 19876..20910 AT2 21517..22053 KR2 22318..22575 ACP2 22867.33555 megAII 22957..27258 Extender Module 3 (3) 22957..24237 KS3 24544..25581 AT3 26230..26733 KR3 (inactive) 26998..27258 ACP3 27313..33312 Extender Module 4 (4) 27393..28590 KS4 28897..29931 AT4 29953..30477 DH4 31396..32244 ER4 32257.32799 KR4 33052.33312 ACP4 33666..43271 megAHI 33780.38120 Extender Module 5 (5) 33780.35027 KS5 35385.36419 AT5 37068.37604 KR5 37860.38120 ACP5 38187..42425 Extender Module 6 (6) 38187.39470 KS6 39795..4081 1 AT6 40398..46641 sequences from cosmid pKOS079-93A 41406..41936 KR6 42168..42425 ACP6 42585..43271 TE 43268.-44344 megCII, TDP-4-keto-6-deoxyglucose 3,4-isomerase 44355..45623 megCIII, TDP-desosaminyl transferase 45620..46591 megBII, TDP-4-keto-6-deoxy-L-glucose 2,3 dehydratase complement(46660..47403) megH, TEII complement(4741 1..47980) megF, C-6 hydroxylase In a specific embodiment, the invention provides an isolated nucleic acid fragment comprising a nucleotide sequence encoding a domain of the megalomicin polyketide synthase or a megalomicin modification enzyme. The isolated nucleic acid fragment can be a DNA or a RNA. Preferably, the isolated nucleic acid fragment is a recombinant DNA compound. A nucleotide sequence that is complementary to the nucleotide sequence encoding a domain of megalomicin PKS or a megalomicin modification enzyme is also provided. The isolated nucleic acid fragment can comprise a single, multiple or all the open reading frame(s) (ORF) of the megalomicin PKS or the megalomicin modification enzyme. Exemplary ORFs of megalomicin PKS include the ORFs of the megAl, megAII and megAIll genes. The isolated nucleic acids of the invention also include nucleic acids that encode one or more domains and one or more modules of the megalomicin PKS. Exemplary domains of the megalomicin PKS include a TE domain, a KS domain, an AT domain, an ACP domain, a KR domain, a DH domain and an ER domain. In a preferred embodiment, the nucleic acid comprises the coding sequence for a loading module, a thioesterase domain, and all six extender modules of the megalomicin PKS. Megalomicin modification enzymes include those enzymes involved in the conversion of 6-DEB into a megalomicin such as the enzymes encoded by megF, meg BV, megCIII, megK, megDI and megG (or megY). Megalomicin modification enzymes also include those enzymes involved in the biosynthesis of mycarose, megosamine or desosamine, which are used as biosynthetic intermediates in the biosynthesis of various megalomicin species and other related polyketides. The enzymes that are involved in biosynthesis of mycarose, megosamine or desosamine are described in Figures 5 and 10. The megalomicin PKS and megalomicin modification enzymes are collectively referred to as megalomicin biosynthetic enzymes; the genes encoding such enzymes are collectively referred to as megalomicin biosynthetic genes; and nucleic acids that comprise a portion of or entire megalomicin biosynthetic genes are collectively referred to as megalomicin biosynthetic nucleic acid(s). In specific embodiments, the megalomicin biosynthetic nucleic acids comprise the sequence of SEQ ID NO: 1 , or the coding regions thereof, or nucleotide sequences encoding, in whole or in part, a megalomicin biosynthetic enzyme protein. The isolated nucleic acids typically consists of at least 25 (continuous) nucleotides, 50 nucleotides, 100 nucleotides, 150 nucleotides, or 200 nucleotides of megalomicin biosynthetic nucleic acid sequence, or a full-length megalomicin biosynthetic coding sequence. In another embodiment, the nucleic acids are smaller than 35, 200, or 500 nucleotides in length. Nucleic acids can be single or double stranded. Nucleic acids that hybridize to or are complementary to the foregoing sequences, in particular the inverse complement to nucleic acids that hybridize to the foregoing sequences (i.e., the inverse complement of a nucleic acid strand has the complementary sequence running in reverse orientation to the strand so that the inverse complement would hybridize without mismatches to the nucleic acid strand) are also provided. In specific aspects, nucleic acids are provided which comprise a sequence complementary to (specifically are the inverse complement of) at least 10, 25, 50, 100, or 200 nucleotides or the entire coding region of a megalomicin biosynthetic gene. The megalomicin biosynthetic nucleic acids provided herein include those with nucleotide sequences encoding substantially the same amino acid sequences as found in native megalomicin biosynthetic enzyme proteins, and those encoding amino acid sequences with functionally equivalent amino acids, as well as megalomicin biosynthetic enzyme derivatives or analogs as described in Section IV. Some regions within the megalomicin PKS genes are highly homologous or identical to one another, as can be readily identified by an analysis of the sequence. The coding sequence for the KS and AT domains of module 2 shares significant identity with the coding sequence for the KS and AT domains of module .6. This sequence homology or identity at the nucleic acid, e.g., DNA, level can render the nucleic acid unstable in certain host cells. To improve the stability of the nucleic acids comprising a portion or the entire megalomicin PKS genes and megalomicin modification enzyme genes, the nucleic acid or DNA sequences can be changed to reduce or abolish the sequence homology or identity. Preferably, the DNA codons of homologous regions within the PKS or the megalomicin modification enzyme coding sequence are changed to reduce or abolish the sequence homology or identity without changing the amino acid sequences encoded by said changed DNA codons (see the examples below). The stability of the nucleic acid or DNA can also be improved by codon changes that reduce or abolish the sequence homology or identity while also changing the amino acid sequence, provided that the amino acid sequence change(s) does not substantially change the desired activity of the encoded megalomicin PKS. Thus, for example, one can simply substitute for the megAIll ORF an ORF from eryAIII, oleAIII, picAIII, or picAIV genes. The recombinant DNA compounds of the invention that encode the megalomicin PKS and modification proteins or portions thereof are useful in a variety of applications. While many of these applications relate to the heterologous expression of the megalomicin biosynthetic genes or the construction of hybrid PKS enzymes, many useful applications involve the natural megalomicin producer Micromonospora megalomicea. For example, one can use the recombinant DNA compounds of the invention to disrupt the megalomicin biosynthetic genes by homologous recombination in Micromonospora megalomicea. The resulting host cell is a preferred host cell for making polyketides modified by oxidation, hydroxylation, glycosylation, and acylation in a manner similar to megalomicin, because the genes that encode the proteins that perform these reactions are of course present in the host cell, and because the host cell does not produce megalomicin that could interfere with production or purification of the polyketide of interest. One illustrative recombinant host cell provided by the present invention expresses a recombinant megalomicin PKS in which the module 1 KS domain is inactivated by deletion or other mutation. In a preferred embodiment, the inactivation is mediated by a change in the KS domain that renders it incapable of binding substrate (called a KS 1 ° mutation). In a particularly preferred embodiment, this inactivation is rendered by a mutation in the codon for the active site cysteine that changes the codon to another codon, such as an alanine codon. Such constructs are especially useful when placed in translational reading frame with extender modules 1 and 2 of a megalomicin or the corresponding modules of another PKS. The utility of these constructs is that host cells expressing, or cell free extracts containing, a PKS comprising the protein encoded thereby can be fed or supplied with N-acylcysteamine thioesters of precursor molecules to prepare a polyketide of interest. See U.S. patent application Serial No. 09/492,773, filed 27 Jan. 2000, and PCT patent publication No. 00/44717, both of which are incorporated herein by reference. Such KS1° constructs of the invention are useful in the production of 13-substituted-megalomicin compounds in Micromonospora megalomicea host cells. Preferred compounds of the invention include those compounds in which the substituent at the 13-position is propyl, vinyl, propargyl, other lower alkyl, and substituted alkyl. In a variant of this embodiment, one can employ a megalomicin PKS in which the ACP domain of module 1 has been rendered inactive. In another embodiment, one can delete the loading domain of the megalomicin PKS and provide monoketide substrates for processing by the remainder of the PKS. The compounds of the invention can also be used to construct recombinant host cells of the invention in which coding sequences for one or more domains or modules of the megalomicin PKS or for another megalomicin biosynthetic gene have been deleted by homologous recombination with the Micromonospora megalomicea chromosomal DNA. Those of skill in the art will appreciate that the compounds used in the recombination process are characterized by their homology with the chromosomal DNA and not by encoding a functional protein due to their intended function of deleting or otherwise altering portions of chromosomal DNA. For this and a variety of other applications, the compounds of the present invention include not only those DNA compounds that encode functional proteins but also those DNA compounds that are complementary or identical to any portion of the megalomicin biosynthetic genes. Thus, the invention provides a variety of modified Micromonospora megalomicea host cells in which one or more of the megalomicin biosynthetic genes have been mutated or disrupted. Transformation systems for M. megalomicea have been described by Hasegawa et al, 1991, J. Bacteriol. / 73:7004-11; and Takada et al, 1994, J. Antibiot. 47 A 167-1170, both of which are incorporated herein by reference. These cells are especially useful when it is desired to replace the disrupted function with a gene product expressed by a recombinant DNA expression vector. While such expression vectors of the invention are described in more detail in the following Section, those of skill in the art will appreciate that the vectors have application to M. megalomicea as well. Such M. megalomicea host cells can be preferred host cells for expressing megalomicin derivatives of the invention. Particularly preferred host cells of this type include those in which the coding sequence for the loading module has been mutated or disrupted, those in which one or more of any of the PKS gene ORFs has been mutated or disrupted, and/or those in which the genes for one or more modification (glycosylation, acylation, hydroxylation) have been mutated or disrupted. While the present invention provides many useful compounds having application to, and recombinant host cells derived from, Micromonospora megalomicea, many important applications of the present invention relate to the heterologous expression of all or a portion of the megalomicin biosynthetic genes in cells other than M. megalomicea, as described in Section V. Section IV: The Megalomicin Biosynthetic Enzymes and Antibodies Recognizing such Enzymes In another specific embodiment, the invention provides a substantially purified polypeptide, which is encoded by a nucleic acid fragment comprising a nucleotide sequence encoding a domain of megalomicin polyketide synthase (PKS) or a megalomicin modification enzyme. The polypeptide can comprise a single domain, multiple domains or a full-length megalomicin PKS or megalomicin modification enzyme. Functional fragments, analogs or derivatives of the megalomicin PKS or megalomicin modification enzyme polypeptides are also provided. Preferably, such fragments, analogs or derivatives can be recognized an antibody raised against a megalomicin PKS or megalomicin modification enzyme. Also preferably, such fragments, analogs or derivatives comprise an amino acid sequence that has at least 60% identity, more preferably at least 90%) identity to their wild type counterparts. An exemplary nucleotide sequence encoding, and the corresponding amino acid sequence of, a megalomicin biosynthetic enzyme is disclosed in SEQ ID NOT . Homologs (e.g., nucleic acids of the above-listed genes of species other than Micromonospora megalomicea) or other related sequences (e.g., paralogs) can be obtained by low, moderate or high stringency hybridization with all or a portion of the particular sequence provided as a probe using methods well known in the art for nucleic acid hybridization and cloning (e.g., as described in Section III) in accordance with the methods of the present invention. The megalomicin biosynthetic enzyme proteins, or domains thereof, of the present invention can be obtained by methods well known in the art for protein purification and recombinant protein expression in accordance with the methods of the present invention. For recombinant expression of one or more of the proteins, the nucleic acid containing all or a portion of the nucleotide sequence encoding the protein can be inserted into an appropriate expression vector, i.e., a vector that contains the necessary elements for the transcription and translation of the inserted protein coding sequence. Transcriptional and translational signals can be supplied by the native promoter for a megalomicin biosynthetic gene and/or flanking regions. A variety of host- vector systems may be utilized to express the protein coding sequence. These include but are not limited to mammalian cell systems infected with virus (e.g. vaccinia virus, adenovirus, and the like); insect cell systems infected with virus (e.g. baculovirus); microorganisms such as yeast containing yeast vectors; or bacteria transformed with bacteriophage, DNA, plasmid DNA, or cosmid DNA. The expression elements of vectors vary in their properties. Depending on the host-vector system utilized, any one of a number of suitable transcription and translation elements may be used. In a specific embodiment, a vector is used that comprises a promoter operably linked to nucleic acid sequences encoding a megalomicin biosynthetic enzyme, or a domain, fragment, derivative or homolog, thereof, one or more origins of replication, and optionally, one or more selectable markers (e.g., an antibiotic resistance gene). Expression vectors containing the sequences of interest can be identified by three general approaches: (a) nucleic acid hybridization, (b) presence or absence of "marker" gene function, and (c) expression of the inserted sequences. In the first approach, megalomicin biosynthetic nucleic acid sequences can be detected by nucleic acid hybridization to probes comprising sequences homologous and complementary to the inserted sequences. In the second approach, the recombinant vector/host system can be identi fied and selected based upon the presence or absence of certain "marker" functions (e.g., binding to an anti-megalomicin biosynthetic enzyme antibody, resistance to antibiotics, occlusion body formation in baculovirus, and the like) caused by insertion of the sequences of interest in the vector. For example, if a megalomicin biosynthetic gene, or portion thereof, is inserted within the marker gene sequence of the vector, recombinants containing the megalomicin biosynthetic gene fragment will be identified by the absence of the marker gene function. In the third approach, recombinant expression vectors can be identified by assaying for the megalomicin biosynthetic gene products expressed by the recombinant vector. Such assays can be based, for example, on the physical or functional properties of the interacting species in in vitro assay systems, e.g., megalomicin synthesis activity, immunoreactivity to antibodies specific for the protein. Once recombinant megalomicin biosynthetic genes or nucleic acids are identified, several methods known in the art can be used to propagate them in accordance with the methods of the present invention. Once a suitable host system and growth conditions have been established, recombinant expression vectors can be propagated and amplified in quantity. As previously described, the expression vectors or derivatives which can be used include, but are not limited to: human or animal viruses such as vaccinia virus or adenovirus; insect viruses such as baculovirus, yeast vectors; bacteriophage vectors such as lambda phage; and plasmid and cosmid vectors. In addition, a host cell strain may be chosen that modulates the expression of the inserted sequences, or modifies or processes the expressed proteins in the specific fashion desired. Expression from certain promoters can be elevated in the presence of certain inducers; thus expression of the genetically-engineered megalomicin biosynthetic enzymes may be controlled. Furthermore, different host cells have characteristic and specific mechanisms for the translational and post- translational processing and modification (e.g. glycosylation, phosphorylation, and the like) of proteins. Appropriate cell lines or host systems can be chosen to ensure the desired modification and processing of the foreign protein is achieved. For example, expression in a bacterial system can be used to produce an unglycosylated core protein, while expression in mammalian cells ensures "native" glycosylation of a heterologous protein. Furthermore, different vector/host expression systems may effect processing reactions to different extent. In particular, megalomicin biosynthetic enzyme derivatives can be made by altering their sequences by substitutions, additions or deletions that provide for functionally equivalent molecules. Due to the degeneracy of nucleotide coding sequences, other DNA sequences which encode substantially the same amino acid sequence as an megalomicin biosynthetic gene can be used in the practice of the present invention. These include but are not limited to nucleotide sequences comprising all or portions of megalomicin biosynthetic genes that are altered by the substitution of different codons that encode the amino acid residue within the sequence, thus producing a silent change. Likewise, the megalomicin biosynthetic enzyme derivatives of the invention include, but are not limited to, those containing, as a primary amino acid sequence, all or part of the amino acid sequence of megalomicin biosynthetic enzymes, including altered sequences in which functionally equivalent amino acid residues are substituted for residues within the sequence resulting in a silent change. For example, one or more amino acid residues within the sequence can be substituted by another amino acid of a similar polarity which acts as a functional equivalent, resulting in a silent alteration. Substitutes for an amino acid within the sequence may be selected from other members of the class to which the amino acid belongs. For example, the nonpolar (hydrophobic) amino acids include alanine, leucine, isoleucine, valine, proline, phenylalanine, tryptophan and methionine. The polar neutral amino acids include glycine, serine, threonine, cysteine, tyrosine, asparagine, and glutamine. The positively charged (basic) amino acids include arginine, lysine and histidine. The negatively charged (acidic) amino acids include aspartic acid and glutamic acid. In a specific embodiment of the invention, the nucleic acids encoding proteins and proteins consisting of or comprising a domain or a fragment of megalomicin biosynthetic enzyme consisting of at least 6 (continuous) amino acids are provided. In other embodiments, the domain or fragment consists of at least 10, 20, 30, 40, or 50 amino acids of a megalomicin biosynthetic enzyme. In specific embodiments, such domains or fragments are not larger than 35, 100 or 200 amino acids. Derivatives .or analogs of megalomicin biosynthetic enzyme include but are not limited to molecules comprising regions that are substantially homologous to megalomicin biosynthetic enzyme in various embodiments, at least 30%, 40%, 50%, 60%, 70%, 80%, 90% or 95% identity over an amino acid sequence of identical size or when compared to an aligned sequence in which the alignment is done by a computer homology program known in the art in accordance with the methods of the present invention or whose encoding nucleic acid is capable of hybridizing to a sequence encoding a megalomicin biosynthetic enzyme under stringent, moderately stringent, or nonstringent conditions. The megalomicin biosynthetic enzyme domains, derivatives and analogs of the invention can be produced by various methods known in the art in accordance with the methods of the present invention. The manipulations which result in their production can occur at the gene or protein level. For example, the cloned megalomicin biosynthetic gene sequence can be modified by any of numerous strategies known in the art (Sambrook et al., 1990, Molecular Cloning, A Laboratory Manual, 2d ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, New York) in accordance with the methods of the present invention. The sequences can be cleaved at appropriate sites with restriction endonuclease(s), followed by further enzymatic modification if desired, isolated, and ligated in vitro. Additionally, the megalomicin biosynthetic enzyme-encoding nucleotide sequence can be mutated in vitro or in vivo, to create and/or destroy translation, initiation, and/or termination sequences, or to create variations in coding regions and/or form new restriction endonuclease sites or destroy pre-existing ones, to facilitate further in vitro modification. Any technique for mutagenesis known in the art can be used in accordance with the methods of the present invention, including but not limited to, chemical mutagenesis and in vitro site-directed mutagenesis (Hutchinson et al., J. Biol. Chem. 253:6551-6558 (1978)), use of TAB® linkers (Pharmacia), and the like. Once a recombinant cell expressing a megalomicin biosynthetic enzyme protein, or a domain, fragment or derivative thereof, is identified, the individual gene product can be isolated and analyzed. This is achieved by assays based on the physical and/or functional properties of the protein, including, but not limited to, radioactive labeling of the product followed by analysis by gel electrophoresis, immunoassay, cross-linking to marker-labeled product, and the like. The megalomicin biosynthetic enzyme proteins may be isolated and purified by standard methods known in the art or recombinant host cells expressing the complexes or proteins in accordance with the methods of the invention, including but not restricted to column chromatography (e.g., ion exchange, affinity, gel exclusion, reversed-phase high pressure, fast protein liquid, and the like), differential centrifugation, differential solubility, or by any other standard technique used for the purification of proteins. Functional properties may be evaluated using any suitable assay known in the art in accordance with the methods of the present invention. Alternatively, once a megalomicin biosynthetic enzyme or its domain or derivative is identified, the amino acid sequence of the protein can be deduced from the nucleotide sequence of the gene which encodes it. As a result, the protein or its domain or derivative can be synthesized by standard chemical methods known in the art in accordance with the methods of the present invention (see Flunkapiller et al, Nature 310: 105-1 1 1 (1984)). Manipulations of megalomicin biosynthetic enzymes may be made at the protein level. Included within the scope of the invention are megalomicin biosynthetic enzyme domains, derivatives or analogs or fragments, which are differentially modified during or after translation, e.g., by glycosylation, acetylation, phosphorylation, amidation, derivatization by known protecting/blocking groups, proteolytic cleavage, linkage to an antibody molecule or other cellular ligand, and the like. Any of numerous chemical modifications may be carried out by known techniques, including but not limited to specific chemical cleavage by cyanogen bromide, trypsin, chymotrypsin, papain, V8 protease, NaBH , acetylation, formylation, oxidation, reduction, metabolic synthesis in the presence of tunicamycin, and the like. In specific embodiments, the megalomicin biosynthetic enzymes are modified to include a fluorescent label. In other specific embodiments, the megalomicin biosynthetic enzyme is modified to have a heterofunctional reagent, such heterofunctional reagents can be used to crosslink the members of the complex. In addition, domains, analogs and derivatives of a megalomicin biosynthetic enzyme can be chemically synthesized. For example, a peptide corresponding to a portion of a megalomicin biosynthetic enzyme, which comprises the desired domain or which mediates the desired activity in vitro can be synthesized by use of a peptide synthesizer. Furthermore, if desired, nonclassical amino acids or chemical amino acid analogs can be introduced as a substitution or addition into the megalomicin biosynthetic enzyme sequence. Non-classical amino acids include but are not limited to the D-isomers of the common amino acids, alpha-amino isobutyric acid, 4-aminobutyric acid, 2-aminobutyric acid, 6-amino hexanoic acid, Aib, 2-amino isobutyric acid, 3-amino propionoic acid, ornithine, norleucine, norvaline, hydroxyproline, sarcosine, citrulline, cysteic acid, t-butylglycine, t-butylalanine, phenylglycine, cyclohexylalanine, β-alanine, fluoro-amino acids, designer amino acids such as β- methyl amino acids, Ca-methyl amino acids, Na-methyl amino acids, and amino acid analogs in general. Furthermore, the amino acid can be D (dextrorotary) or L (levorotary). In cases where natural products are suspected of being mutant or are isolated from new species, the amino acid sequence of the megalomicin biosynthetic enzyme isolated from the natural source, as well as those expressed in vitro , or from synthesized expression vectors in vivo or in vitro, can be determined from analysis of the DNA sequence, or alternatively, by direct sequencing of the isolated protein. Such analysis may be performed by manual sequencing or through use of an automated amino acid sequenator. The megalomicin biosynthetic enzyme proteins may also be analyzed by hydrophilicity analysis (Hopp and Woods, Proc. Natl. Acad. Sci. USA 78:3824- 3828 (1981)). A hydrophilicity profile can be used to identify the hydrophobic and hydrophilic regions of the proteins, and help predict their orientation in designing substrates for experimental manipulation, such as in binding experiments, antibody synthesis, and the like. Secondary structural analysis can also be done to identify regions of the megalomicin biosynthetic enzyme that assume specific structures (Chou and Fasman, Biochemistry j3:222-23 (1974)). Manipulation, translation, secondary structure prediction, hydrophilicity and hydrophobicity profiles, open reading frame prediction and plotting, and determination of sequence homologies, can be accomplished using computer software programs available in the art. Other methods of structural analysis including but not limited to X-ray crystallography (Engstrom, Biochem. Exp. Biol. jT :7-13 (1974)), mass spectroscopy and gas chromatography (Methods in Protein Science, J. Wiley and Sons, New York, 1997), and computer modeling (Fletterick and Zoller, eds., 1986, Computer Graphics and Molecular Modeling, In: Current Communications in Molecular Biology, Cold Spring Harbor Laboratory, Cold Spring Harbor Press, New York) can also be employed. The invention also provides an antibody, or a fragment or derivative thereof, which immuno-specifically binds to a domain of megalomicin polyketide synthase (PKS) or a megalomicin modification enzyme. In a specific embodiment, an antibody which immuno-specifically binds to a domain of the megalomicin biosynthetic enzyme encoded by a nucleic acid that hybridizes to a nucleic acid having the nucleotide sequence set forth in the SEQ. ID NO: 1 , or a fragment or derivative of said antibody containing the binding domain thereof is provided. Preferably, the antibody is a monoclonal antibody. The megalomicin biosynthetic enzyme protein and domains, fragments, homologs and derivatives thereof may be used as immunogens to generate antibodies which immunospecifically bind such immunogens. Such antibodies include but are not limited to polyclonal, monoclonal, chimeric, single chain, Fab fragments, and an Fab expression library. Various procedures known in the art may be used for the production of polyclonal antibodies to a megalomicin biosynthetic enzyme protein of the invention, its domains, derivatives, fragments or analogs in accordance with the methods of the present invention. For production of the antibody, various host animals can be immunized by injection with the native megalomicin biosynthetic enzyme protein or a synthetic version, or a derivative of the foregoing, such as a cross-linked megalomicin biosynthetic enzyme. Such host animals include but are not limited to rabbits, mice, rats, and the like. Various adjuvants can be used to increase the immunological response, depending on the host species, and include but are not limited to Freund's (complete and incomplete), mineral gels such as aluminum hydroxide, surface active substances such as lysolecithin, pluronic polyols, polyanions, peptides, oil emulsions, dinitrophenol, and potentially useful human adjuvants such as bacille Calmette-Guerin (BCG) and corynebacterium parvum. For preparation of monoclonal antibodies directed towards a megalomicin biosynthetic enzyme or domains, derivatives, fragments or analogs thereof, any technique that provides for the production of antibody molecules by continuous cell lines in culture may be used. Such techniques include but are not restricted to the hybridoma technique originally developed by Kohler and Milstein (Nature 256:495-497 (1975)), the trioma technique, the human B-cell hybridoma technique (Kozbor et al., Immunology Today 4:72 (1983)), and the EBV hybridoma technique to produce human monoclonal antibodies (Cole et al., in Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, Inc., pp. 77-96 (1985)). In an additional embodiment, monoclonal antibodies can be produced in germ-free animals (WO89/12690). Human antibodies may be used and can be obtained by using human hybridomas (Cote et al., Proc. Natl. Acad. Sci. USA 80:2026-2030 (1983)) or by transforming human B cells with EBV virus in vitro (Cole et al., in Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, Inc., pp. 77-96 (1985)). Techniques developed for the production of "chimeric antibodies" (Morrison et al., Proc. Natl. Acad. Sci. USA 81 :6851-6855 (1984); Neuberger et al., Nature 312:604-608 (1984); Takeda et al., Nature 314:452-454 (1985)) by splicing the genes from a mouse antibody molecule specific for the megalomicin biosynthetic enzyme protein together with genes from a human antibody molecule of appropriate biological activity can be used; such antibodies are within the scope of this invention. Techniques described for the production of single chain antibodies (U.S. patent 4,946,778) can be adapted to produce megalomicin biosynthetic enzyme- specific single chain antibodies. An additional embodiment utilizes the techniques described for the construction of Fab expression libraries (Huse et al., Science 246:1275-1281 (1989)) to allow rapid and easy identification of monoclonal Fab fragments with the desired specificity for megalomicin biosynthetic enzyme, or domains, derivatives, or analogs thereof. Non-human antibodies can be "humanized" by known methods (see, e.g., U.S. Patent No. 5,225,539). Antibody fragments that contain the idiotypes of a megalomicin biosynthetic enzyme can be generated by techniques known in the art in accordance with the methods of the present invention. For example, such fragments include but are not limited to: the F(ab')2 fragment which can be produced by pepsin digestion of the antibody molecule; the Fab' fragments that can be generated by reducing the disulfide bridges of the F(ab')2 fragment, the Fab fragments that can be generated by treating the antibody molecular with papain and a reducing agent, and Fv fragments. In the production of antibodies, screening for the desired antibody can be accomplished by techniques known in the art in accordance with the methods of the present invention, e.g. , ELISA (enzyme-linked immunosorbent assay). To select antibodies specific to a particular domain of the megalomicin biosynthetic enzyme, one may assay generated hybridomas for a product that binds to the fragment of a megalomicin biosynthetic enzyme that contains such a domain. The foregoing antibodies can be used in methods known in the art relating to the localization and/or quantitation of megalomicin biosynthetic enzyme proteins, e.g., for imaging these proteins or measuring levels thereof in samples, in accordance with the methods of the present invention. Section V: Heterologous Expression of the Megalomicin Biosynthetic Genes In one important embodiment, the invention provides methods for the heterologous expression of one or more of the megalomicin biosynthetic genes and recombinant DNA expression vectors useful in the method. For purposes of the invention, any host cell other than Micromonospora megalomicea is a heterologous host cell. Thus, included within the scope of the invention in addition to isolated nucleic acids encoding domains, modules, or proteins of the megalomicin PKS and modification enzymes, are recombinant expression vectors that include such nucleic acids. The term expression vector refers to a nucleic acid that can be introduced into a host cell or cell-free transcription and translation system. An expression vector can be maintained permanently or transiently in a cell, whether as part of the chromosomal or other DNA in the cell or in any cellular compartment, such as a replicating vector in the cytoplasm. An expression vector also comprises a promoter that drives expression of an RNA, which typically is translated into a polypeptide in the cell or cell extract. For efficient translation of RNA into protein, the expression vector also typically contains a ribosome-binding site sequence positioned upstream of the start codon of the coding sequence of the gene to be expressed. Other elements, such as enhancers, secretion signal sequences, transcription termination sequences, and one or more marker genes by which host cells containing the vector can be identified and/or selected, may also be present in an expression vector. Selectable markers, i.e., genes that confer antibiotic resistance or sensitivity, are preferred and confer a selectable phenotype on transformed cells when the cells are grown in an appropriate selective medium. The various components of an expression vector can vary widely, depending on the intended use of the vector and the host cell(s) in which the vector is intended to replicate or drive expression. Expression vector components suitable for the expression of genes and maintenance of vectors in E. coli, yeast, Streptomyces, and other commonly used cells are widely known and commercially available. For example, suitable promoters for inclusion in the expression vectors of the invention include those that function in eucaryotic or procaryotic host cells. Promoters can comprise regulatory sequences that allow for regulation of expression relative to the growth of the host cell or that cause the expression of a gene to be turned on or off in response to a chemical or physical stimulus. For E. coli and certain other bacterial host cells, promoters derived from genes for biosynthetic enzymes, antibiotic-resistance conferring enzymes, and phage proteins can be used and include, for example, the galactose, lactose (lac), maltose, tryptophan (trp), beta-lactamase (bid), bacteriophage lambda PL, and T5 promoters. In addition, synthetic promoters, such as the tac promoter (U.S. Patent No. 4,551 ,433), can also be used. Thus, recombinant expression vectors contain at least one expression system, which, in turn, is composed of at least a portion of the megalomicin PKS and/or other megalomicin biosynthetic gene coding sequences operably linked to a promoter and optionally termination sequences that operate to effect expression of the coding sequence in compatible host cells. The host cells are modified by transformation with the recombinant DNA expression vectors of the invention to contain the expression system sequences either as extrachromosomal elements or integrated into the chromosome. The resulting host cells of the invention are useful in methods to produce PKS and post-PKS modification enzymes as well as polyketides and antibiotics and other useful compounds derived therefrom. Preferred host cells for purposes of selecting vector components for expression vectors of the present invention include fungal host cells such as yeast and procaryotic host cells such as E. coli and Streptomyces, but mammalian host cells can also be used. In hosts such as yeasts, plants, or mammalian cells that ordinarily do not produce polyketides, it may be necessary to provide, also typically by recombinant means, suitable holo-ACP synthases to convert the recombinantly produced PKS to functionality. Provision of such enzymes is described, for example, in PCT publication Nos. WO 97/13845 and 98/27203, each of which is incorporated herein by reference. Particularly preferred host cells for purposes of the present invention are Streptomyces and Saccharopolyspora host cells, as discussed in greater detail below. In a preferred embodiment, the expression vectors of the invention are used to construct a heterologous recombinant Streptomyces host cell that expresses a recombinant PKS of the invention. Streptomyces is a convenient host for expressing polyketides, because polyketides are naturally produced in certain Streptomyces species, and Streptomyces cells generally produce the precursors needed to form the desired polyketide. Those of skill in the art will recognize that, if a Streptomyces host cell produces any portion of a PKS enzyme or produces a polyketide modification enzyme, the recombinant vector need drive expression of only those genes constituting the remainder of the desired PKS enzyme or other polyketide-modifying enzymes. Thus, such a vector may comprise only a single ORF, with the desired remainder of the polypeptides constituting the PKS provided by the genes on the host cell chromosomal DNA. If a Streptomyces or other host cell ordinarily produces polyketides, it may be desirable to modify the host so as to prevent the production of endogenous polyketides prior to its use to express a recombinant PKS of the invention. Such modified hosts include S. coelicolor CH999 and similarly modified S. lividans described in U.S. Patent No. 5,672,491, and PCT publication Nos. WO 95/08548 and WO 96/40968, incorporated herein by reference. In such hosts, it may not be necessary to provide enzymatic activities for all of the desired post-translational modifications of the enzymes that make up the recombinantly produced PKS, because the host naturally expresses such enzymes. In particular, these hosts generally contain holo-ACP synthases that provide the phosphopantotheinyl residue needed for functionality of the PKS. The invention provides a wide variety of expression vectors for use in Streptomyces. The replicating expression vectors of the present invention include, for example and without limitation, those that comprise an origin of replication from a low copy number vector, such as SCP2* (see Hopwood et al, Genetic Manipulation of Streptomyces: A Laboratory manual (The John Innes Foundation, Norwich, U.K., 1985); Lydiate et al, 1985, Gene 35: 223-235; and Kieser and Melton, 1988, Gene 65: 83-91 , each of which is incorporated herein by reference), SLP1.2 (Thompson et al, 1982, Gene 20: 51-62, incorporated herein by reference), and pSG5(ts) (Muth et al, 1989, Mol. Gen. Genet. 219: 341-348, and Bierman et al, 1992, Gene 116: 43-49, each of which is incorporated herein by reference), or a high copy number vector, such as pIJlOl and pJVl (see Katz et al, 1983, J. Gen. Microbiol. 129: 2703-2714; Vara et al, 1989, J. Bacteriol. 171: 5782-5781 ; and Servin-Gonzalez, 1993, Plasmid 30: 131-140, each of which is incorporated herein by reference). For non-replicating and integrating vectors and generally for any vector, it is useful to include at least an E. coli origin of replication, such as from pUC, plP, pll, and pBR. For phage based vectors, the phage phiC31 and its derivative KC515 can be employed (see Hopwood et al, supra). Also, plasmid pSET152, plasmid pSAM, plasmids pSElOl and pSE21 1, all of which integrate site-specifically in the chromosomal DNA of S. lividans, can be employed for purposes of the present invention. The Streptomyces recombinant expression vectors of the invention typically comprise one or more selectable markers, including antibiotic resistance conferring genes selected from the group consisting of the ermE (confers resistance to erythromycin and lincomycin), tsr (confers resistance to thiostrepton), aadA (confers resistance to spectinomycin and streptomycin), aacC4 (confers resistance to apramycin, kanamycin, gentamicin, geneticin (G418), and neomycin), hyg (confers resistance to hygromycin), and vph (confers resistance to viomycin) resistance conferring genes. Alternatively, several polyketides are naturally colored, and this characteristic can provide a built-in marker for identifying cells. Megalomicins are currently produced only by the relatively genetically intractable host Micromonospora megalomicinea. This bacteria has not been commonly used in the fermentation industry for the large-scale production of antibiotics, and methods for high level production of megalomicin and its analogs are needed. In contrast, the streptomycete bacteria have been widely used for almost 50 years and are excellent hosts for production of megalomicin and its analogs. Streptomyces lividans and S. coelicolor have been developed for the expression of heterologous PKS systems. These organisms can stably maintain cloned heterologous PKS genes, express them at high levels under controlled conditions, and modify the corresponding PKS proteins (e.g., phosphopantotheinylation) so that they are capable of production of the polyketide they encode. Furthermore, these hosts contain the necessary pathways to produce the substrates required for polyketide synthesis; e.g. propionyl-CoA and methylmalonyl-CoA. A wide variety of cloning and expression vectors are available for these hosts, as are methods for the introduction and stable maintenance of large segments of foreign DNA. Relative to Micromonospora spp., S. lividans and S. coelicolor grow well on a number of media and have been adapted for high level production of polyketides in fermentors. If production levels are low, a number of rational approaches are available to improve yield (see Hosted and Baltz, 1996, Trends Biotechnol. 14(7):245-50, incorporated herein by reference). Empirical methods to increase the titers of these macrolides, long since proven effective for numerous bacterial polyketides, can also be employed. Preferred Streptomyces host cell/vector combinations of the invention include S. coelicolor CH999 and S. lividans K4-114 host cells, which have been modified so as not to produce the polyketide actinorhodin, and expression vectors derived from the pRMl and pRM5 vectors, as described in U.S. Patent Nos. 5,830,750 and 6,022,731 and U.S. patent application Serial No. 09/181,833, filed 28 Oct. 1998, each of which is incorporated herein by reference. These vectors are particularly preferred in that they contain promoters compatible with numerous and diverse Streptomyces spp. Particularly useful promoters for Streptomyces host cells include those from PKS gene clusters that result in the production of polyketides as secondary metabolites, including promoters from aromatic (Type II) PKS gene clusters. Examples of Type II PKS gene cluster promoters are act gene promoters and tcm gene promoters; an example of a Type I PKS gene cluster promoter are the promoters of the spiramycin PKS genes and DEBS genes. The present invention also provides the megalomicin biosynthetic gene promoters in recombinant form. These promoters can be used to drive expression of the megalomicin biosynthetic genes or any other coding sequence of interest in host cells in which the promoter functions, particularly Micromonospora megalomicea and generally any Streptomyces species. As described above, particularly useful control sequences are those that alone or together with suitable regulatory systems activate expression during transition from growth to stationary phase in the vegetative mycelium. The promoter contained in the aforementioned plasmid pRM5, i.e., the actl/actlll promoter pair and the actII-ORF4 activator gene, is particularly preferred. Other useful Streptomyces promoters include without limitation those from the ermE gene and the melCl gene, which act constitutively, and the tipA gene and the merA gene, which can be induced at any growth stage. In addition, the T7 RNA polymerase system has been transferred to Streptomyces and can be employed in the vectors and host cells of the invention. In this system, the coding sequence for the T7 RNA polymerase is inserted into a neutral site of the chromosome or in a vector under the control of the inducible merA promoter, and the gene of interest is placed under the control of the T7 promoter. As noted above, one or more activator genes can also be employed to enhance the activity of a promoter. Activator genes in addition to the actII-ORF4 gene described above include dnrl, redD, anάptpA genes (see U.S. patent application Serial No. 09/181,833, supra). To provide a preferred host cell and vector for purposes of the invention, the megalomicin biosynthetic genes are placed on a recombinant expression vector and transferred to the non-macrolide producing hosts Streptomyces lividans K4- 114 and S. coelicolor CH999. Transformation of S. lividans K4-1 14 or S. coelicolor CH999 with this expression vector results in a strain which produces detectable amounts of megalomicin as determined by analysis of extracts by LC/MS. As noted above, the present invention also provides recombinant DNA compounds in which the encoded megalomicin module 1 KS domain is inactivated (the KS1° mutation). The introduction into Streptomyces lividans or S. coelicolor of a recombinant expression vector of the invention that encodes a megalomicin PKS with a KS1° domain produces a host cell useful for making polyketides by a process known as diketide feeding. The resulting host cells can be fed or supplied with N-acylcysteamine thioesters of precursor molecules to prepare megalomicin derivatives. Such cells of the invention are especially useful in the production of 13-substituted-6-deoxyerythronolide B compounds in recombinant host cells. Preferred compounds of the invention include those compounds in which the substituent at the 13-position is propyl, vinyl, propargyl, other lower alkyl, and substituted alkyl. In a preferred embodiment, the meg PKS is produced from a recombinant construct in which the megAIll gene has been altered to abolish the regions of identical coding sequence it otherwise shares with the megAI gene, or a hybrid PKS is employed in which the megAIll gene product has been replaced by the oleAIII gene product. Recombinant oleAIII genes are described in, for example, PCT patent publication No. 00/026349 and U.S. patent application Serial No. 09/428,517, filed 28 Oct. 1999, both of which are incorporated herein by reference. The recombinant host cells of the invention can express all of the megalomicin biosynthetic genes or only a subset of the same. For example, if only the genes for the megalomicin PKS are expressed in a host cell that otherwise does not produce polyketide modifying enzymes that can act on the polyketide produced, then the host cell produces unmodified polyketides, called macrolide aglycones. Such macrolide aglycones can be hydroxylated and glycosylated by adding them to the fermentation of a strain such as, for example, Streptomyces antibioticus or Saccharopolyspora erythraea, that contains the requisite modification enzymes. There are a wide variety of diverse organisms that can modify macrolide aglycones to provide compounds with, or that can be readily modified to have, useful activities. For example, as shown in Figure 5, Saccharopolyspora erythraea can convert 6-dEB to a variety of useful compounds. The erythronolide 6-dEB is converted by the eryF gene product to erythronolide B, which is, in turn, glycosylated by the eryBV gene product to obtain 3-O-mycarosylerythronolide B, which contains L-mycarose at C-3. The eryCIII gene product then converts this compound to erythromycin D by glycosylation with D-desosamine at C-5. Erythromycin D, therefore, differs from 6-dEB through glycosylation and by the addition of a hydroxyl group at C-6. Erythromycin D can be converted to erythromycin B in a reaction catalyzed by the eryG gene product by methylating the L-mycarose residue at C-3. Erythromcyin D is converted to erythromycin C by the addition of a hydroxyl group at C-12 in a reaction catalyzed by the eryK gene product. Erythromycin A is obtained from erythromycin C by methylation of the mycarose residue in a reaction catalyzed by the eryG gene product. The unmodified megalomicin compounds provided by the present invention, such as, for example, the 6-dEB or 6-dEB analogs, produced in Streptomyces lividans, can be provided to cultures of S. erythraea and converted to the corresponding derivatives of erythromycins A, B, C, and D in accordance with the procedure provided in the examples below. To ensure that only the desired compound is produced, one can use an S. erythraea eryA mutant that is unable to produce 6- dEB but can still carry out the desired conversions (Weber et al, 1985, J. Bacteriol. 164(\): 425-433). Also, one can employ other mutant strains, such as eryB, eryC, eryG, and/or eryK mutants, or mutant strains having mutations in multiple genes, to accumulate a preferred compound. The conversion can also be carried out in large fermentors for commercial production. Moreover, there are other useful organisms that can be employed to hydroxylate and/or glycosylate the compounds of the invention. As described above, the organisms can be mutants unable to produce the polyketide normally produced in that organism, the fermentation can be carried out on plates or in large fermentors, and the compounds produced can be chemically altered after fermentation. Thus, Streptomyces venezuelae, which produces picromycin, contains enzymes that can transfer a desosaminyl group to the C-5 hydroxyl and a hydroxyl group to the C-12 position. In addition, S. venezuelae contains a glucosylation activity that glucosylates the 2' -hydroxyl group of the desosamine sugar. This latter modification reduces antibiotic activity, but the glucosyl residue is removed by enzymatic action prior to release of the polyketide from the cell. Another organism, S. narbonensis, contains the same modification enzymes as S. venezuelae, except the C-12 hydroxylase. Thus, the present invention provides the compounds produced by hydroxylation and glycosylation of the macrolide aglycones of the invention by action of the enzymes endogenous to S. narbonensis and S. venezuelae. Other organisms suitable for making compounds of the invention include Micromonospora megalomicea (discussed above), Streptomyces antibioticus, S. fradiae, and S. thermotolerans. S. antibioticus produces oleandomycin and contains enzymes that hydroxylate the C-6 and C-12 positions, glycosylate the C-3 hydroxyl with oleandrose and the C-5 hydroxyl with desosamine, and form an epoxide at C-8-C-8a. S. fradiae contains enzymes that glycosylate the C-5 hydroxyl with mycaminose and then the 4 '-hydroxyl of mycaminose with mycarose, forming a disaccharide. S. thermotolerans contains the same activities as S. fradiae, as well as acylation activities. Thus, the present invention provides the compounds produced by hydroxylation and glycosylation of the macrolide aglycones of the invention by action of the enzymes endogenous to S. antibioticus, S. fradiae, and S. thermotolerans. The present invention also provides methods and genetic constructs for producing the glycosylated and/or hydroxylated compounds of the invention directly in the host cell of interest. Thus, the recombinant genes of the invention, which include recombinant megAI, megAII, and megAIll genes with one or more deletions and/or insertions, including replacements ϊ megA gene fragment with a gene fragment from a heterologous PKS gene (as discussed in the next Section), can be included on expression vectors suitable for expression of the encoded gene products in Saccharopolyspora erythraea, Streptomyces antibioticus, S. venezuelae, S. narbonensis, Micromonospora megalomicea, S. fradiae, and S. thermotolerans. A number of erythromycin high-producing strains of Saccharopolyspora erythraea and Streptomyces fradiae have been developed, and in a preferred embodiment, the megalomicin PKS and/or other megalomicin biosynthetic genes are introduced into such strains (or erythromycin non-producing mutants thereof) to provide the corresponding modified megalomicin compounds in high yields. Those of skill in the art will appreciate that S. erythraea contains the desosamine and mycarose biosynthetic and transfer genes as well as DEBS, which, as noted above, makes the same macrolide aglycone, 6-dEB, as the megalomicin PKS. S. erythraea does not make megosamine or its corresponding transferase gene, and does not contain the acylation gene of Micromonospora megalomicea. Finally, the S. erythraea eryG gene product converts mycarose to cladinose, which does not occur in M. megalomicea. Thus, the present invention provides a wide variety of S. erythraea recombinant host cells, including, for example, those that contain: (i) wild-type erythromycin biosynthetic genes with recombinant megosamine biosynthetic and transfer genes, with and without megalomicin acylation genes; (ii) wild-type erythromycin biosynthetic genes except eryG, with recombinant megosamine biosynthetic and transfer genes, with and without megalomicin acylation genes; and (iii) as in (i) and (ii), except that the eryA genes are inactive or deleted and recombinant megA genes have been introduced. The invention provides other S. erythraea strains as well, including those in which any one or more of the erythromycin biosynthetic genes have been deleted or otherwise rendered inactive and in which at least one megalomicin biosynthetic gene has been introduced. For example, the present invention enables one to express the megosamine genes in a Saccharopolyspora erythraea eryG mutant in which the erythromycin C made by this mutant is converted to megalomicin A. Alternatively, one could use an erythromycin C high -producing strain of S. erythraea in biotransformation methods in which the erythromycin C is fed to a Streptomyces lividans strain carrying only the megosamine biosynthesis and glycosyltransferase genes. As another alternative, one could use a strain of S. lividans that carries suitable erythromycin production genes along with the daunosamine biosynthesis genes plus geneX and geneY of Figure 5, or all of the megosamine biosynthesis genes, to produce megalomicin A. All or some of the megalomicin gene cluster can be easily cloned under control of a suitable promoter in pCK7 or pSET 152 either in one or two plasmids and introduced into the Saccharopolyspora erythraea eryG mutant. The actll- ORF4/- ct/p system and the phiC31/ t system in pSET function well in this organism (see Rowe et al, 1998, Gene, 216:215-23, incorporated herein by reference). Alternatively, the megosamine biosynthesis genes are introduced into Streptomyces lividans on the same plasmids and the production of megalomicin A or its precursor mediated by bioconversion, done by feeding erythronolide B, 3- alpha-mycarosylerythronolide B, erythromycin D or erythromycin C to the S. lividans strain. Lack of adequate resistance to megalomicin A in S. erythraea or S. lividans is not expected, because both organisms have MLS resistance genes (ermE and mgt/lrm, respectively), which confer resistance to several 14-membered macrolides (see Cundliffe, 1989, Annu. Rev. Microbiol. 43:207-33; Jenkins and Cundliffe, 1991, Gene 108:55-62; and Cundliffe, 1992, Gene, 775:75-84, each of which is incorporated herein by reference). One can also readily determine the level of resistance of the S. erythraea eryG mutant and the S. lividans host cells to megalomicin A, both in plate tests and in liquid medium. One can repeat the bioconversion method using an eryG mutant of a high erythromycin A producing S. erythraea strain (or an eryB or eryC mutant, as necessary) to determine the level at which megalomicin A can be produced. Furthermore, if experience shows that high level megalomicin A production requires a higher level of resistance to this macrolide than present in S. erythraea or S. lividans, the necessary megalomicin self- resistance genes will be cloned from M. megalomicea and moved into either one of the heterologous hosts. This will be straightforward work since self- resistance genes are usually found in the cluster of macrolide biosynthesis genes and can be identified by their homology to known macrolide resistance genes and(or) by the resistance phenotype they impart to a strain that normally is sensitive. Alternatively, geneX and geneY (Figure 5) can be added to cassettes containing the relevant daunosamine (dnm) biosynthesis genes (Figure 5) to provide the ability to make TDP-megosamine in vivo and attach it to an erythromycin algycone. The TDP-daunosamine biosynthesis genes can be re- cloned from Streptomyces peucetius on two compatible and mutually selectable plasmids. When an S. lividans strain containing these two plasmids and the dnmS gene for TDP-daunosamine glycosyltransferase is grown in the presence of added epsilon-rhodomycinone, its glycoside with L-daunosamine, called rhodomycin D, is produced in good yield. Thus, bioconversion of one of the erythromycins to megalomicin A should be observed when geneX and geneY are present. One can construct all five combination - the two /V-dimethyltransferase genes and the three glycosyltransferase genes - to discriminate geneX and geneY from those connected with mycarose and desosamine biosynthesis and attachment in the megalomicin pathway. Because the timing of megosamine addition is unknown, one can test erythronolide B, 3-alpha-mycarosylerythronolide B, erythromycin D and erythromycin C as substrates provided to a strain that expresses the megosamine biosynthetic and transferase genes. There is need to test the C3'" and(or) C4'" acylated metabolites like megalomicin Cl, because these metabolites are made from megalomicin A and not the converse, based on the precedents in the biosynthesis of tylosin (see Arisawa et αl, 1994, Appl. Environ. Microbiol. 60: 2657-2661), carbomycin (see Epp et αl, 1989, Gene 55:293-301), and midecamycin (see Hara and Hutchinson, 1992, J. Bacteriol. 174, 5141 -5144). If C-6 glycosylation of erythronolide B or 3-alpha-mycarosylerythronolide B (Figure 5) happens before addition of desosamine to C-5, then the erythromycin genes might not be able to complete formation of megalomicin A from some mono or diglycoside if the erythromycin glycosyltransferases cannot tolerate a C-6 glycoside. Although unexpected, such an outcome could be circumvented in accordance with the methods of the invention by cloning further megalomicin biosynthesis genes into the appropriate S. erythraea background or into S. lividans - specifically, the necessary deoxysugar biosynthesis and attachment genes - to create a recombinant strain that produces megalomicin A. The acyltransferase gene that adds acetate or propionate to the C3'" or C4'" positions of mycarose in megalomicin B, Cl and C2 (Figure 3) is contained within the cosmids of the invention and can be identified by scanning the sequence data for the megalomicin gene cluster to locate homologs of car E and mdmB or their acyA homologs from the tylosin producer. The carE and acyA genes govern C4'" acylation in the carbomycin and tylosin pathway, respectively. The megalomicin homolog has the equivalent function in megalomicin biosynthesis (but is specific for C3'" and C4'" acylation). The gene can be cloned under control of a suitable promoter and introduced into S. lividans to produce the desired acyl derivative of megalomicin A. Alternatively, introduction of the carE gene can form megalomicin B. This gene can be cloned from the carbomycin, spiramycin or tylosin producers. If the amount of megalomicin produced by an S. erythraea or S. lividans or other recombinant host cell is less than desired, yield can be improved by optimizing the growth medium and fermentation conditions, by increasing expression of the gene(s) that appear to be rate limiting, based on the level of pathway intermediates that are accumulated by the recombinant strain constructed, and by reconstructing the ery, dnm, and megalomicin biosynthesis genes on vectors like pSETl 52 that can be integrated into the genome to provide a stabler recombinant strain for strain improvement. In another embodiment, the present invention provides recombinant vectors encoding one or more of the megosamine, desosamine, and mycarose biosynthetic and transfer genes and heterologous host cells comprising those vectors. In this embodiment of the invention, the heterologous host cell is typically a cell that is unable to produce the sugar and transfer it to a polyketide unless the vector of the invention is introduced. For example, neither Streptomyces lividans nor S. coelicolor is naturally capable of making megosamine, desosamine, or mycarose or transferring those moieties to a polyketide. However, the present invention provides recombinant Streptomyces lividans and S. coelicolor host cells that are capable of making megosamine, desosamine, and/or mycarose and transferring those moieties to a polyketide. Moreover, additional recombinant gene products can be expressed in the host cell to improve production of a desired polyketide. As but one non-limiting example, certain of the recombinant PKS proteins of the invention may produce a polyketide other than or in addition to the predicted polyketide, because the polyketide is cleaved from the PKS by the thioesterase (TE) domain in module 6 prior to processing by other domains on the PKS, in particular, any KR, DH, and/or ER domains in module 6. The production of the predicted polyketide can be increased in such instances by deleting the TE domain coding sequences from the gene and, optionally, expressing the TE domain as a separate protein. See Gokhale et al, Feb. 1999, "Mechanism and specificity of the terminal thioesterase domain from the erythromycin polyketide synthase," Chem. & Biol. 6: 117-125, incorporated herein by reference. Thus, in one important aspect, the present invention provides methods, expression vectors, and recombinant host cells that enable the production of megalomicin and hydroxylated and glycosylated derivatives of megalomicin in heterologous host cells. The present invention also provides methods for making a wide variety of polyketides derived in part from the megalomicin PKS or other biosynthetic genes, as described in the following Section. Section VI: Hybrid PKS Genes The present invention provides recombinant DNA compounds encoding each of the domains of each of the modules of the megalomicin PKS as well as the other megalomicin biosynthetic enzymes. The availability of these compounds permits their use in recombinant procedures for production of desired portions of the megalomicin PKS fused to or expressed in conjunction with all or a portion of a heterologous PKS and, optionally, one or more polyketide modification enzymes. These compounds also permit the modification of polyketides with the various megalomicin modification enzymes. The resulting hybrid PKS can then be expressed in a host cell to produce a desired polyketide or modified form thereof. Thus, in accordance with the methods of the invention, a portion of the megalomicin biosynthetic gene coding sequence that encodes a particular activity can be isolated and manipulated, for example, to replace the corresponding region in a different modular PKS gene or modification enzyme gene. In addition, coding sequences for individual proteins, modules, domains, and portions thereof of the megalomicin PKS can be ligated into suitable expression systems and used to produce the portion of the protein encoded. The resulting protein can be isolated and purified or can may be employed in situ to effect polyketide synthesis. Depending on the host for the recombinant production of the domain, module, protein, or combination of proteins, suitable control sequences such as promoters, termination sequences, enhancers, and the like are ligated to the nucleotide sequence encoding the desired protein in the construction of the expression vector, as described above. In one important embodiment, the invention thus provides hybrid PKS enzymes and the corresponding recombinant DNA compounds that encode those hybrid PKS enzymes. For purposes of the invention, a hybrid PKS is a recombinant PKS that comprises all or part of one or more extender modules, loading module, and/or thioesterase/cyclase domain of a first PKS and all or part of one or more extender modules, loading module, and/or thioesterase/cyclase domain of a second PKS. In one preferred embodiment, the first PKS is most but not all of the megalomicin PKS, and the second PKS is only a portion of a non- megalomicin PKS. An illustrative example of such a hybrid PKS includes a megalomicin PKS in which the megalomicin PKS loading module has been replaced with a loading module of another PKS. Another example of such a hybrid PKS is a megalomicin PKS in which the AT domain of extender module 3 is replaced with an AT domain that binds only malonyl CoA. In another preferred embodiment, the first PKS is most but not all of a non-megalomicin PKS, and the second PKS is only a portion of the megalomicin PKS. An illustrative example of such a hybrid PKS includes a rapamycin PKS in which an AT specific for malonyl CoA is replaced with the AT from the megalomicin PKS specific for methylmalonyl CoA. Other illustrative hybrid PKSs of the invention are described below. Those of skill in the art will recognize that all or part of either the first or second PKS in a hybrid PKS of the invention need not be isolated from a naturally occurring source. For example, only a small portion of an AT domain determines its specificity. See PCT patent application No. WO US99/15047, and Lau et al, infra, incorporated herein by reference. The state of the art in DNA synthesis allows the artisan to construct de novo DNA compounds of size sufficient to construct a useful portion of a PKS module or domain. Thus, the desired derivative coding sequences can be synthesized using standard solid phase synthesis methods such as those described by Jaye et al, 1984, J Biol. Chem. 259: 6331, and instruments for automated synthesis are available commercially from, for example, Applied Biosystems, Inc. For purposes of the invention, such synthetic DNA compounds are deemed to be a portion of a PKS. With this general background regarding hybrid PKSs of the invention, one can better appreciate the benefit provided by the DNA compounds of the invention that encode the individual domains, modules, and proteins that comprise the megalomicin PKS. As described above, the megalomicin PKS is comprised of a loading module, six extender modules composed of a KS, AT, ACP, and zero, one, two, or three KR, DH, and ER domains, and a thioesterase domain. The DNA compounds of the invention that encode these domains individually or in combination are useful in the construction of the hybrid PKS encoding DNA compounds of the invention. For example, a DNA compound of the invention that encodes an extender module or portion of an extender module is useful in the construction of a coding sequence that encodes a protein subcomponent of a PKS. The DNA compound of the invention that comprises a coding sequence of a PKS subunit protein is useful in the construction of an expression vector that drives expression of the subunit in a host cell that expresses the other subunits and so produces a functional PKS. The recombinant DNA compounds of the invention that encode the loading module of the megalomicin PKS and the corresponding polypeptides encoded thereby are useful for a variety of applications. In one embodiment, a DNA compound comprising a sequence that encodes the megalomicin PKS loading module is inserted into a DNA compound that comprises the coding sequence for one or more heterologous PKS extender modules. The resulting construct, in which the coding sequence for the loading module of the heterologous PKS is replaced by that for the coding sequence of the megalomicin PKS loading module provides a novel PKS. Examples include the DEBS, rapamycin, FK-506, FK-520, rifamycin, and avermectin PKS coding sequences. In another embodiment, a DNA compound comprising a sequence that encodes the megalomicin PKS loading module is inserted into a DNA compound that comprises the coding sequence for the megalomicin PKS or a recombinant megalomicin PKS that produces a megalomicin derivative. In another embodiment, a portion of the loading module coding sequence is utilized in conjuction with a heterologous coding sequence. In this embodiment, the invention provides, for example, replacing the methylmalonyl CoA (propionyl) specific AT with a malonyl CoA (acetyl), ethylmalonyl CoA (butyryl), or other CoA specific AT. In addition, the AT and/or ACP can be replaced by another AT and/or another ACP or an inactivated KS, such as a KS , an AT, and/or another ACP. The resulting heterologous loading module coding sequence can be utilized in conjunction with a coding sequence for a PKS that synthesizes megalomicin, a megalomicin derivative, or another polyketide. The recombinant DNA compounds of the invention that encode the first extender module of the megalomicin PKS and the corresponding polypeptides encoded thereby are useful for a variety of applications. In one embodiment, a DNA compound comprising a sequence that encodes the megalomicin PKS first extender module is inserted into a DNA compound that comprises the coding sequence for a heterologous PKS. The resulting construct, in which the coding sequence for a module of the heterologous PKS is either replaced by that for the first extender module of the megalomicin PKS or the latter is merely added to coding sequences for modules of the heterologous PKS, provides a novel PKS coding sequence. In another embodiment, a DNA compound comprising a sequence that encodes the first extender module of the megalomicin PKS is inserted into a DNA compound that comprises coding sequences for the megalomicin PKS or a recombinant megalomicin PKS that produces a megalomicin derivative. In another embodiment, a portion or all of the first extender module coding sequence is utilized in conjunction with other PKS coding sequences to create a hybrid module. In this embodiment, the invention provides, for example, replacing the methylmalonyl CoA specific AT with a malonyl CoA, ethylmalonyl CoA, or 2-hydroxymalonyl CoA specific AT; deleting (which includes inactivating) the KR; inserting a DH or a DH and ER; and/or replacing the KR with another KR, a DH and KR, or a DH, KR, and ER. In addition, the KS and/or ACP can be replaced with another KS and/or ACP. In each of these replacements or insertions, the heterologous KS, AT, DH, KR, ER, or ACP coding sequence can originate from a coding sequence for another module of the megalomicin PKS, from a gene for a PKS that produces a polyketide other than megalomicin, or from chemical synthesis. The resulting heterologous first extender module coding sequence can be utilized in conjunction with a coding sequence for a PKS that synthesizes megalomicin, a megalomicin derivative, or another polyketide. Those of skill in the art will recognize, however, that deletion of the KR domain of extender module 1 or insertion of a DH domain or DH and KR domains into extender module 1 will prevent the typical cyclization of the polyketide at the hydroxyl group created by the KR if such hybrid module is employed as a first extender module in a hybrid PKS or is otherwise involved in producing a portion of the polyketide at which cyclization is to occur. Such deletions or insertions can be useful, however, to create linear molecules or to induce cyclization at another site in the molecule. As noted above, the invention also provides recombinant PKSs and recombinant DNA compounds and vectors that encode such PKSs in which the KS domain of the first extender module has been inactivated. Such constructs are typically expressed in translational reading frame with the first two extender modules on a single protein, with the remaining modules and domains of a megalomicin, megalomicin derivative, or hybrid PKS expressed as one or more, typically two, proteins to form the multi-protein functional PKS. The utility of these constructs is that host cells expressing, or cell free extracts containing, the PKS encoded thereby can be fed or supplied with N-acylcysteamine thioesters of precursor molecules to prepare megalomicin derivative compounds. See U.S. patent application Serial No. 09/492,733, filed 27 Jan. 2000, and PCT publication Nos. WO 00/44717, 99/03986 and 97/02358, each of which is incorporated herein by reference. The recombinant DNA compounds of the invention that encode the second extender module of the megalomicin PKS and the corresponding polypeptides encoded thereby are useful for a variety of applications. In one embodiment, a DNA compound comprising a sequence that encodes the megalomicin PKS second extender module is inserted into a DNA compound that comprises the coding sequence for a heterologous PKS. The resulting construct, in which the coding sequence for a module of the heterologous PKS is either replaced by that for the second extender module of the megalomicin PKS or the latter is merely added to coding sequences for the modules of the heterologous PKS, provides a novel PKS. In another embodiment, a DNA compound comprising a sequence that encodes the second extender module of the megalomicin PKS is inserted into a DNA compound that comprises the coding sequences for the megalomicin PKS or a recombinant megalomicin PKS that produces a megalomicin derivative. In another embodiment, a portion or all of the second extender module coding sequence is utilized in conjunction with other PKS coding sequences to create a hybrid module. In this embodiment, the invention provides, for example, replacing the methylmalonyl CoA specific AT with a malonyl CoA, ethylmalonyl CoA, or 2-hydroxymalonyl CoA specific AT; deleting (or inactivating) the KR; replacing the KR with a KR, a KR and a DFI, or a KR, DH, and ER; and/or inserting a DH or a DH and an ER. In addition, the KS and/or ACP can be replaced with another KS and/or ACP. In each of these replacements or insertions, the heterologous KS, AT, DH, KR, ER, or ACP coding sequence can originate from a coding sequence for another module of the megalomicin PKS, from a coding sequence for a PKS that produces a polyketide other than megalomicin, or from chemical synthesis. The resulting heterologous second extender module coding sequence can be utilized in conjunction with a coding sequence from a PKS that synthesizes megalomicin, a megalomicin derivative, or another polyketide. The recombinant DNA compounds of the invention that encode the third extender module of the megalomicin PKS and the corresponding polypeptides encoded thereby are useful for a variety of applications. In one embodiment, a DNA compound comprising a sequence that encodes the megalomicin PKS third extender module is inserted into a DNA compound that comprises the coding sequence for a heterologous PKS. The resulting construct, in which the coding sequence for a module of the heterologous PKS is either replaced by that for the third extender module of the megalomicin PKS or the latter is merely added to coding sequences for the modules of the heterologous PKS, provides a novel PKS. In another embodiment, a DNA compound comprising a sequence that encodes the third extender module of the megalomicin PKS is inserted into a DNA compound that comprises coding sequences for the megalomicin PKS or a recombinant megalomicin PKS that produces a megalomicin derivative. In another embodiment, a portion or all of the third extender module coding sequence is utilized in conjunction with other PKS coding sequences to create a hybrid module. In this embodiment, the invention provides, for example, replacing the methylmalonyl CoA specific AT with a malonyl CoA, ethylmalonyl CoA, or 2-hydroxymalonyl CoA specific AT; deleting the inactive KR; and/or replacing the KR with an active KR, or a KR and DH, or a KR, DH, and ER. In addition, the KS and/or ACP can be replaced with another KS and/or ACP. In each of these replacements or insertions, the heterologous KS, AT, DH, KR, ER, or ACP coding sequence can originate from a coding sequence for another module of the megalomicin PKS, from a gene for a PKS that produces a polyketide other than megalomicin, or from chemical synthesis. The resulting heterologous third extender module coding sequence can be utilized in conjunction with a coding sequence for a PKS that synthesizes megalomicin, a megalomicin derivative, or another polyketide. The recombinant DNA compounds of the invention that encode the fourth extender module of the megalomicin PKS and the corresponding polypeptides encoded thereby are useful for a variety of applications. In one embodiment, a DNA compound comprising a sequence that encodes the megalomicin PKS fourth extender module is inserted into a DNA compound that comprises the coding sequence for a heterologous PKS. The resulting construct, in which the coding sequence for a module of the heterologous PKS is either replaced by that for the fourth extender module of the megalomicin PKS or the latter is merely added to coding sequences for the modules of the heterologous PKS, provides a novel PKS. In another embodiment, a DNA compound comprising a sequence that encodes the fourth extender module of the megalomicin PKS is inserted into a DNA compound that comprises coding sequences for the megalomicin PKS or a recombinant megalomicin PKS that produces a megalomicin derivative. In another embodiment, a portion of the fourth extender module coding sequence is utilized in conjunction with other PKS coding sequences to create a hybrid module. In this embodiment, the invention provides, for example, replacing the methylmalonyl CoA specific AT with a malonyl CoA, ethylmalonyl CoA, or 2-hydroxymalonyl CoA specific AT; deleting or inactivating any one, two, or all three of the ER, DH, and KR; and/or replacing any one, two, or all three of the ER, DH, and KR with either a KR, a DH and KR, or a KR, DH, and ER. In addition, the KS and/or ACP can be replaced with another KS and/or ACP. In each of these replacements or insertions, the heterologous KS, AT, DH, KR, ER, or ACP coding sequence can originate from a coding sequence for another module of the megalomicin PKS (except for the DH and ER domains), from a coding sequence for a PKS that produces a polyketide other than megalomicin, or from chemical synthesis. The resulting heterologous fourth extender module coding sequence can be utilized in conjunction with a coding sequence for a PKS that synthesizes megalomicin, a megalomicin derivative, or another polyketide. The recombinant DNA compounds of the invention that encode the fifth extender module of the megalomicin PKS and the corresponding polypeptides encoded thereby are useful for a variety of applications. In one embodiment, a DNA compound comprising a sequence that encodes the megalomicin PKS fifth extender module is inserted into a DNA compound that comprises the coding sequence for a heterologous PKS. The resulting construct, in which the coding sequence for a module of the heterologous PKS is either replaced by that for the fifth extender module of the megalomicin PKS or the latter is merely added to coding sequences for the modules of the heterologous PKS, provides a novel PKS. In another embodiment, a DNA compound comprising a sequence that encodes the fifth extender module of the megalomicin PKS is inserted into a DNA compound that comprises the coding sequence for the megalomicin PKS or a recombinant megalomicin PKS that produces a megalomicin derivative. In another embodiment, a portion or all of the fifth extender module coding sequence is utilized in conjunction with other PKS coding sequences to create a hybrid module. In this embodiment, the invention provides, for example, replacing the methylmalonyl CoA specific AT with a malonyl CoA, ethylmalonyl CoA, or 2-hydroxymalonyl CoA specific AT; deleting (or inactivating) the KR; inserting a DH or a DH and ER; and/or replacing the KR with another KR, a DH and KR, or a DFI, KR, and ER. In addition, the KS and/or ACP can be replaced with another KS and/or ACP. In each of these replacements or insertions, the heterologous KS, AT, DH, KR, ER, or ACP coding sequence can originate from a coding sequence for another module of the megalomicin PKS, from a coding sequence for a PKS that produces a polyketide other than megalomicin, or from chemical synthesis. The resulting heterologous fifth extender module coding sequence can be utilized in conjunction with a coding sequence for a PKS that synthesizes megalomicin, a megalomicin derivative, or another polyketide. The recombinant DNA compounds of the invention that encode the sixth extender module of the megalomicin PKS and the corresponding polypeptides encoded thereby are useful for a variety of applications. In one embodiment, a DNA compound comprising a sequence that encodes the megalomicin PKS sixth extender module is inserted into a DNA compound that comprises the coding sequence for a heterologous PKS. The resulting construct, in which the coding sequence for a module of the heterologous PKS is either replaced by that for the sixth extender module of the megalomicin PKS or the latter is merely added to coding sequences for the modules of the heterologous PKS, provides a novel PKS. In another embodiment, a DNA compound comprising a sequence that encodes the sixth extender module of the megalomicin PKS is inserted into a DNA compound that comprises the coding sequences for the megalomicin PKS or a recombinant megalomicin PKS that produces a megalomicin derivative. In another embodiment, a portion or all of the sixth extender module coding sequence is utilized in conjunction with other PKS coding sequences to create a hybrid module. In this embodiment, the invention provides, for example, replacing the methylmalonyl CoA specific AT with a malonyl CoA, ethylmalonyl CoA, or 2-hydroxymalonyl CoA specific AT; deleting or inactivating the KR or replacing the KR with another KR, a KR and DH, or a KR, DH, and an ER; and/or inserting a DH or a DH and ER. In addition, the KS and/or ACP can be replaced with another KS and/or ACP. In each of these replacements or insertions, the heterologous KS, AT, DFI, KR, ER, or ACP coding sequence can originate from a coding sequence for another module of the megalomicin PKS, from a coding sequence for a PKS that produces a polyketide other than megalomicin, or from chemical synthesis. The resulting heterologous sixth extender module coding sequence can be utilized in conjunction with a coding sequence for a PKS that synthesizes megalomicin, a megalomicin derivative, or another polyketide. The sixth extender module of the megalomicin PKS is followed by a thioesterase domain. This domain is important in the cyclization of the polyketide and its cleavage from the PKS. The present invention provides recombinant DNA compounds that encode hybrid PKS enzymes in which the megalomicin PKS is fused to a heterologous thioesterase or a heterologous PKS is fused to the megalomicin PKS thioesterase. Thus, for example, a thioesterase domain coding sequence from another PKS gene can be inserted at the end of the sixth (or other final) extender module coding sequence in recombinant DNA compounds of the invention or the megalomicin PKS thioesterase can be similarly fused to a heterologous PKS. Recombinant DNA compounds encoding this thioesterase domain are useful in constructing DNA compounds that encode the megalomicin PKS, a PKS that produces a megalomicin derivative, and a PKS that produces a polyketide other than megalomicin or a megalomicin derivative. Thus, the hybrid modules of the invention are incorporated into a PKS to provide a hybrid PKS of the invention. A hybrid PKS of the invention can result not only: (i) from fusions of heterologous domain (where heterologous means the domains in a module are derived from at least two different naturally occurring modules) coding sequences to produce a hybrid module coding sequence contained in a PKS gene whose product is incorporated into a PKS, but also: (ii) from fusions of heterologous modules (where heterologous module means two modules are adjacent to one another that are not adjacent to one another in naturally occurring PKS enzymes) coding sequences to produce a hybrid coding sequence contained in a PKS gene whose product is incorporated into a PKS, (iii) from expression of one or more megalomicin PKS genes with one or more non-megalomicin PKS genes, including both naturally occurring and recombinant non-megalomicin PKS genes, and (iv) from combinations of the foregoing. Various hybrid PKSs of the invention illustrating these various alternatives are described herein. An example of a hybrid PKS comprising fused modules results from fusion of the loading module of either the DEBS PKS or the narbonolide PKS (see PCT patent application No. US99/1 1814, incorporated herein by reference) with extender modules 1 and 2 of the megalomicin PKS to produce a hybrid megAI gene. Co-expression of either one of these two hybrid megAI genes with the megAIl and megAIll genes in suitable host cells, such as Streptomcyes lividans, results in expression of a hybrid PKS of the invention that produces 6- deoxyerythronolide B (the polyketide product of the natural megA genes) in recombinant host cells. Co-expression of either one of these two hybrid megAI genes with the eryAII and eryAIII genes similarly results in the production of 6- dEB, while co-expression with the analogous narbonolide PKS genes, picAII, picAIII and picAIV, results in the production of 3-deoxy-3-oxo-6-dEB (3-keto-6- dEB), useful in the production of ketolides, compounds with potent anti-bacterial activity. Another example of a hybrid PKS comprising a hybrid module is prepared by co-expressing the megAI and megAIl genes with a megAIll hybrid gene encoding extender module 5 and the KS and AT of extender module 6 of the megalomicin PKS fused to the ACP of module 6 and the TE of the narbonolide PKS. The resulting hybrid PKS of the invention produces 3-keto-6-dEB. This compound can also be prepared by a recombinant megalomicin derivative PKS of the invention in which the KR domain of module 6 of the megalomicin PKS has been deleted. Moreover, the invention provides hybrid PKSs in which not only the above changes have been made but also the AT domain of module 6 has been replaced with a malonyl-specific AT. These hybrid PKSs produce 2-desmethyl-3- deoxy-3-oxo-6-dEB, a useful intermediate in the preparation of 2-desmethyl ketolides, compounds with potent antibiotic activity. Another illustrative example of a hybrid PKS includes the hybrid PKS of the invention resulting only from the latter change in the hybrid PKS just described. Thus, co-expression of the megAI and megAIl genes with a hybrid megAIll gene in which the AT domain of module 6 has been replaced by a malonyl-specific AT results in the expression of a hybrid PKS that produces 2- desmethyl-6-dEB in recombinant host cells. This compound is a useful intermediate for making 2-desmethyl erythromycins in recombinant host cells of the invention, as well as for making 2-desmethyl semi -synthetic ketolides. While many of the hybrid PKSs described above are composed primarily of megalomicin PKS proteins, those of skill in the art recognize that the present invention provides many different hybrid PKSs, including those composed of only a small portion of the megalomicin PKS. For example, the present invention provides a hybrid PKS in which a hybrid eryAI gene that encodes the megalomicin PKS loading module fused to extender modules 1 and 2 of DEBS is coexpressed with the eryAII and eryAIII genes. The resulting hybrid PKS produces 6-dEB, the product of the native DEBS. When the construct is expressed in Saccharopolyspora erythraea host cells (either via chromosomal integration in the chromosome or via a vector that encodes the hybrid PKS), the resulting recombinant host cell of the invention produces erythromycins. Another illustrative example is the hybrid PKS of the invention composed of the megAI and eryAII and eryAIII gene products. This construct is also useful in expressing erythromycins in Saccharopolyspora erythraea host cells. In a preferred embodiment, the S. erythraea host cells are eryAI mutants that do not produce 6- deoxyerythronolide B. Another example is the hybrid PKS of the invention composed of the products of the picAI and picAII genes (the two proteins that comprise the loading module and extender modules 1 - 4, inclusive, of the narbonolide PKS) and the megAIll gene. The resulting hybrid PKS produces the macrolide aglycone 3- hydroxy-narbonolide in Streptomyces lividans host cells and the corresponding erythromycins in Saccharopolyspora erythraea host cells. Each of the foregoing hybrid PKS enzymes of the invention, and the hybrid PKS enzymes of the invention generally, can be expressed in a host cell that also expresses a functional oleP gene product. The oleP gene encodes an oleandomycin modi fication enzyme, and expression of the gene together with a hybrid PKS of the invention provides the compounds of the invention in which a C-8 hydroxyl, a C-8a or C-8-C-8a epoxide is present. Recombinant methods for manipulating modular PKS genes to make hybrid PKS enzymes are described in U.S. Patent Nos. 5,672,491 ; 5,843,718; 5,830,750; and 5,712,146; and in PCT publication Nos. 98/49315 and 97/02358, each of which is incorporated herein by reference. A number of genetic engineering strategies have been used with DEBS to demonstrate that the structures of polyketides can be manipulated to produce novel natural products, primarily analogs of the erythromycins (see the patent publications referenced supra and Hutchinson, 1998, Curr Opin Microbiol. 7:319-329, and Baltz, 1998, Trends Microbiol. 6:76-83, incorporated herein by reference). Because of the similar activity of the megalomicin PKS and DEBS (both PKS enzymes produce the macrolide aglycone 6-dEB), these methods can be readily applied to the recombinant megalomicin PKS genes of the invention. These techniques include: (i) deletion or insertion of modules to control chain length, (ii) inactivation of reduction/dehydration domains to bypass beta- carbon processing steps, (iii) substitution of AT domains to alter starter and extender units, (iv) addition of reduction/dehydration domains to introduce catalytic activities, and (v) substitution of ketoreductase KR domains to control hydroxyl stereochemistry. In addition, engineered blocked mutants of DEBS have been used for precursor directed biosynthesis of analogs that incorporate synthetically derived starter units. For example, more than 100 novel polyketides were produced by engineering single and combinatorial changes in multiple modules of DEBS. Hybrid PKS enzymes based on DEBS with up to three catalytic domain substitutions were constructed by cassette mutagenesis, in which various DEBS domains were replaced with domains from the rapamycin PKS (see Schweke et al, 1995, Proc. Nat. Acad. Sci. USA 92, 7839-7843, incorporated herein by reference) or one more of the DEBS KR domains was deleted. Functional single domain replacements or deletions were combined to generate DEBS enzymes with double and triple catalytic domain substitutions (see McDaniel et al, 1999, Proc. Nat. Acad. Sci. USA 96, 1846-1851, incorporated herein by reference). By providing the analogous megalomicin/rapamycin hybrid PKS enzymes, the present invention provides alternative means to make these polyketides. Methods for generating libraries of polyketides have been greatly improved by cloning PKS genes as a set of three or more mutually selectable plasmids, each carrying a different wild-type or mutant PKS gene, then introducing all possible combinations of the plasmids with wild-type, mutant, and hybrid PKS coding sequences into the same host (see U.S. patent application Serial No. 60/129,731 , filed 16 Apr. 1999, and PCT Pub. No. 98/27203, each of which is incorporated herein by reference). This method can also incorporate the use of a KS1° mutant, which by mutational biosynthesis can produce polyketides made from diketide starter units (see Jacobsen et al, 1997, Science 277, 367-369, incorporated herein by reference), as well as the use of a truncated gene that leads to 12-membered macrolides or an elongated gene that leads to 16-membered ketolides. Moreover, by utilizing in addition one or more vectors that encode glycosyl biosynthesis and transfer genes, such as those of the present invention for megosamine, desosamine, oleandrose, cladinose, and/or mycarose (in any combination), a large collection of glycosylated polyketides can be prepared. The following Table lists references describing illustrative PKS genes and corresponding enzymes that can be utilized in the construction of the recombinant hybrid PKSs and the corresponding DNA compounds that encode them of the invention. Also presented are various references describing tailoring enzymes and corresponding genes that can be employed in accordance with the methods of the invention. Avermectin U.S. Pat. No. 5,252,474 to Merck. MacNeil et al, 1993, Industrial Microorganisms: Basic and Applied Molecular Genetics, Baltz, Hegeman, & Skatrud, eds. (ASM), pp. 245-256, A Comparison of the Genes Encoding the Polyketide Synthases for Avermectin, Erythromycin, and Nemadectin. MacNeil et al, 1992, Gene 115: 1 19-125, Complex Organization of the Streptomyces avermitilis genes encoding the avermectin polyketide synthase. Candicidin (FR008) Hu et α/., 1994, Mol. Microbiol. 14: 163-172. Epothilone PCT Pub. No. 00/031247 to Kosan. Erythromycin PCT Pub. No. 93/13663 to Abbott. US Pat. No. 5,824,513 to Abbott. Donadio et al, 1991, Science 252:675-9. Cortes et al, 8 Nov. 1990, Nature 348: 176-8, An unusually large multifunctional polypeptide in the erythromycin producing polyketide synthase of Saccharopolyspora erythraea. Glycosylation Enzymes PCT Pub. No. 97/23630 to Abbott. FK-506 Motamedi et al, 1998, The biosynthetic gene cluster for the macrolactone ring of the immunosuppressant FK506, Ewr. J. biochem. 256: 528-534. Motamedi et al, 1997, Structural organization of a multifunctional polyketide synthase involved in the biosynthesis of the macrolide immunosuppressant FK506, Eur. J. Biochem. 244: 74-80. Methyltransferase US 5,264,355, issued 23 Nov. 1993, Methylating enzyme from Streptomyces MA6858. 31-O-desmethyl-FK506 methyltransferase. Motamedi et al, 1996, Characterization of methyltransferase and hydroxylase genes involved in the biosynthesis of the immunosuppressants FK506 and FK520, J Bacteriol. 178: 5243-5248. FK-520 PCT Pub. No. 00/20601 to Kosan. See also Nielsen et al, 1991, Biochem. 30:5789-96 (enzymology of pipecolate incorporation). Lovastatin U.S. Pat. No. 5,744,350 to Merck. Narbomycin (and Picromycin) PCT Pub. No. WO US99/61599 to Kosan. Nemadectin MacNeil et al, 1993, supra. Niddamycin Kakavas et al, 1997, Identification and characterization of the niddamycin polyketide synthase genes from Streptomyces caelestis, J. Bacteriol. 179: 7515- 7522. Oleandomycin Swan et al, 1994, Characterization of a Streptomyces antibioticus gene encoding a type I polyketide synthase which has an unusual coding sequence, Mol. Gen. Genet. 242: 358-362. PCT Pub. No. 00/026349 to Kosan. Olano et al, 1998, Analysis of a Streptomyces antibioticus chromosomal region involved in oleandomycin biosynthesis, which encodes two glycosyltransferases responsible for glycosylation of the macrolactone ring, Mol. Gen. Genet. 259(3): 299-308. Platenolide EP Pub. No. 791 ,656 to Lilly. Rapamycin Schwecke et al, Aug. 1995, The biosynthetic gene cluster for the polyketide rapamycin, Proc. Natl. Acad. Sci. USA 92:7839-7843. Aparicio et al, 1996, Organization of the biosynthetic gene cluster for rapamycin in Streptomyces hygroscopic us: analysis of the enzymatic domains in the modular polyketide synthase, Gene 169: 9-16. Rifamycin August et al, 13 Feb. 1998, Biosynthesis of the ansamycin antibiotic rifamycin: deductions from the molecular analysis of the rif biosynthetic gene cluster of Amy colatops is mediterranei S669, Chemistry & Biology, 5(2): 69-79. Soraphen U.S. Pat. No. 5,716,849 to Novartis. Schupp et al, 1995, J. Bacteriology 177: 3673-3679. A Sorangium cellulosum (Myxobacterium) Gene Cluster for the Biosynthesis of the Macrolide Antibiotic Soraphen A: Cloning, Characterization, and Homology to Polyketide Synthase Genes from Actinomycetes. Spiramycin U.S. Pat. No. 5,098,837 to Lilly. Activator Gene U.S. Pat. No. 5,514,544 to Lilly. Tylosin EP Pub. No. 791,655 to Lilly. Kuhstoss et al, 1996, Gene 753:231-6., Production of a novel polyketide through the construction of a hybrid polyketide synthase. U.S. Pat. No. 5,876,991 to Lilly. Tailoring enzymes Merson-Davies and Cundliffe, 1994, Mol. Microbiol. 13: 349-355. Analysis of five tylosin biosynthetic genes from the tylBA region of the Streptomyces fradiae genome. As the above Table illustrates, there are a wide variety of PKS genes that serve as readily available sources of DNA and sequence information for use in constructing the hybrid PKS-encoding DNA compounds of the invention. In constructing hybrid PKSs of the invention, certain general methods may be helpful. For example, it is often beneficial to retain the framework of the module to be altered to make the hybrid PKS. Thus, if one desires to add DH and ER functionalities to a module, it is often preferred to replace the KR domain of the original module with a cognate KR, DH, and ER domain-containing segment from another module, instead of merely inserting DH and ER domains. One can alter the stereochemical specificity of a module by replacement of the KS domain with a KS domain from a module that specifies a different stereochemistry. See Lau et al., 1999, "Dissecting the role of acyltransferase domains of modular polyketide synthases in the choice and stereochemical fate of extender units" Biochemistry 38(5): 1643- 1651, incorporated herein by reference. One can alter the specificity of an AT domain by changing only a small segment of the domain. See Lau et al., supra. One can also take advantage of known linker regions in PKS proteins to link modules from two different PKSs to create a hybrid PKS. See Gokhale et al, 16 Apr. 1999, Dissecting and Exploiting Intermodular Communication in Polyketide Synthases", Science 284: 482-485, incorporated herein by reference. The hybrid PKS-encoding DNA compounds of the invention can be and often are hybrids of more than two PKS genes. Even where only two genes are used, there are often two or more modules in the hybrid gene in which all or part of the module is derived from a second (or third) PKS gene. Thus, as one illustrative example, the invention provides a hybrid PKS that contains the naturally occurring loading module and thioesterase domain as well as extender modules one, two, four, and six of the megalomicin PKS and further contains hybrid or heterologous extender modules three and five. Flybrid or heterologous extender modules three and five contain AT domains specific for malonyl CoA and derived from, for example, the rapamycin PKS genes. The invention also provides libraries of PKS genes, PKS proteins, and ultimately, of polyketides, that are constructed by generating modifications in the megalomicin PKS so that the protein complexes produced have altered activities in one or more respects and thus produce polyketides other than the natural product of the PKS. Novel polyketides may thus be prepared, or polyketides in general prepared more readily, using this method. By providing a large number of different genes or gene clusters derived from a naturally occurring PKS gene cluster, each of which has been modified in a different way from the native cluster, an effectively combinatorial library of polyketides can be produced as a result of the multiple variations in these activities. As will be further described below, the metes and bounds of this embodiment of the invention can be described on the polyketide, protein, and the encoding nucleotide sequence levels. As described above, a modular PKS "derived from" the megalomicin or other naturally occurring PKS includes a modular PKS (or its corresponding encoding gene(s)) that retains the scaffolding of the utilized portion of the naturally occurring gene. Not all modules need be included in the constructs; however, the constructs can also comprise more than six modules. On the constant scaffold, at least one enzymatic activity is mutated, deleted, replaced, or inserted so as to alter the activity of the resulting PKS relative to the original (native) PKS. Alteration results when these activities are deleted or are replaced by a different version of the activity, or simply mutated in such a way that a polyketide other than the natural product results from these collective activities. This occurs because there has been a resulting alteration of the starter unit and/or extender unit, stereochemistry, chain length or cyclization, and/or reductive or dehydration cycle outcome at a corresponding position in the product polyketide. Where a deleted activity is replaced, the origin of the replacement activity may come from a corresponding activity in a different naturally occurring PKS or from a different region of the megalomicin PKS. Any or all of the megalomicin PKS genes may be included in the derivative or portions of any of these may be included, but the scaffolding of a functional PKS protein is retained in whatever derivative is constructed. The derivative preferably contains a thioesterase activity from the megalomicin or another PKS. Thus, a PKS derived from the megalomicin PKS includes a PKS that contains the scaffolding of all or a portion of the megalomicin PKS. The derived PKS also contains at least two extender modules that are functional, preferably three extender modules, and more preferably four or more extender modules, and most preferably six extender modules. The derived PKS also contains mutations, deletions, insertions, or replacements of one or more of the activities of the functional modules of the megalomicin PKS so that the nature of the resulting polyketide is altered at both the protein and DNA sequence levels. Particular preferred embodiments include those wherein a KS, AT, or ACP domain has been deleted or replaced by a version of the activity from a different PKS or from another location within the same PKS. Also preferred are derivatives where at least one non-condensation cycle enzymatic activity (KR, DH, or ER) has been deleted or added or wherein any of these activities has been mutated so as to change the structure of the polyketide synthesized by the PKS. Conversely, also included within the definition of a PKS derived from the megalomicin PKS are functional non-megalomicin PKS modules or their encoding genes wherein at least one domain or coding sequence therefor of a megalomicin PKS module has been inserted. Exemplary is the use of the megalomicin AT for extender module 2, which accepts a methylmalonyl CoA extender unit rather than malonyl CoA, to replace a malonyl specific AT in another PKS. Other examples include insertion of portions of non-condensation cycle enzymatic activities or other regions of megalomicin synthase activity into a heterologous PKS at both the DNA and protein levels. Thus, there are at least five degrees of freedom for constructing a hybrid PKS in terms of the polyketide that will be produced. First, the polyketide chain length is determined by the number of extender modules in the PKS, and the present invention includes hybrid PKSs that contain 6, as wells as fewer or more than 6, extender modules. Second, the nature of the carbon skeleton of the PKS is determined by the specificities of the acyl transferases that determine the nature of the extender units at each position, e.g., malonyl, methylmalonyl, ethylmalonyl, or other substituted malonyl. Third, the loading module specificity also has an effect on the resulting carbon skeleton of the polyketide. The loading module may use a different starter unit, such as acetyl, butyryl, and the like. As noted above, another method for varying loading module specificity involves inactivating the KS activity in extender module 1 (KS1) and providing alternative substrates, called diketides, that are chemically synthesized analogs of extender module 1 diketide products, for extender module 2. This approach was illustrated in PCT publication Nos. 97/02358 and 99/03986, incorporated herein by reference, wherein the KS1 activity was inactivated through mutation. Fourth, the oxidation state at various positions of the polyketide will be determined by the dehydratase and reductase portions of the modules. This will determine the presence and location of ketone and alcohol moieties and C-C double bonds or C-C single bonds in the polyketide. Finally, the stereochemistry of the resulting polyketide is a function of three aspects of the synthase. The first aspect is related to the AT/KS specificity associated with substituted malonyls as extender units, which affects stereochemistry only when the reductive cycle is missing or when it contains only a ketoreductase, as the dehydratase would abolish chirality. Second, the specificity of the ketoreductase may determine the chirality of any beta-OH. Finally, the enoylreductase specificity for substituted malonyls as extender units may influence the stereochemistry when there is a complete KR/DH/ER available. Thus, the modular PKS systems generally and the megalomicin PKS system particularly permit a wide range of polyketides to be synthesized. As compared to the aromatic PKS systems, the modular PKS systems accept a wider range of starter units, including aliphatic monomers (acetyl, propionyl, butyryl, isovaleryl, and the like.), aromatics (aminohydroxybenzoyl), alicyclics (cyclohexanoyl), and heterocyclics (thiazolyl). Certain modular PKSs have relaxed specificity for their starter units (Kao et al., 1994, Science, supra). Modular PKSs also exhibit considerable variety with regard to the choice of extender units in each condensation cycle. The degree of beta-ketoreduction following a condensation reaction can be altered by genetic manipulation (Donadio et al. , 1991, Science, supra; Donadio et al, 1993, Proc. Natl. Acad. Sci. USA 90: 7119- 7123). Likewise, the size of the polyketide product can be varied by designing mutants with the appropriate number of modules (Kao et al. , 1994, J. Am. Chem. Soc. 77. .1 1612-1 1613). Lastly, modular PKS enzymes are particularly well known for generating an impressive range of asymmetric centers in their products in a highly controlled manner. The polyketides, antibiotics, and other compounds produced by the methods of the invention are typically single stereoisomeric forms. Although the compounds of the invention can occur as mixtures of stereoisomers, it may be beneficial in some instances to generate individual stereoisomers. Thus, the combinatorial potential within modular PKS pathways based on any naturally occurring modular, such as the megalomicin, PKS scaffold is virtually unlimited. While hybrid PKSs are most often produced by "mixing and matching" portions of PKS coding sequences, mutations in DNA encoding a PKS can also be used to introduce, alter, or delete an activity in the encoded polypeptide. Mutations can be made to the native sequences using conventional techniques. The substrates for mutation can be an entire cluster of genes or only one or two of them; the substrate for mutation may also be portions of one or more of these genes. Techniques for mutation include preparing synthetic oligonucleotides including the mutations and inserting the mutated sequence into the gene encoding a PKS subunit using restriction endonuclease digestion. See, e.g., Kunkel, 1985, Proc. Natl. Acad. Sci. USA 82: 448; Geisselsoder et al, 1987, BioTechniques 5:786. Alternatively, the mutations can be effected using a mismatched primer (generally 10-20 nucleotides in length) that hybridizes to the native nucleotide sequence, at a temperature below the melting temperature of the mismatched duplex. The primer can be made specific by keeping primer length and base composition within relatively narrow limits and by keeping the mutant base centrally located. See Zoller and Smith, 1983, Methods Enzymol 100:468. Primer extension is effected using DNA polymerase, the product cloned, and clones containing the mutated DNA, derived by segregation of the primer extended strand, selected. Identification can be accomplished using the mutant primer as a hybridization probe. The technique is also applicable for generating multiple point mutations. See, e.g., Dalbie-McFarland et al, 1982, Proc. Natl. Acad. Sci. USA 79: 6409. PCR mutagenesis can also be used to effect the desired mutations. Random mutagenesis of selected portions of the nucleotide sequences encoding enzymatic activities can also be accomplished by several different techniques known in the art, e.g., by inserting an ohgonucleotide linker randomly into a plasmid, by irradiation with X-rays or ultraviolet light, by incorporating incorrect nucleotides during in vitro DNA synthesis, by error-prone PCR mutagenesis, by preparing synthetic mutants, or by damaging plasmid DNA in vitro with chemicals, in accordance with the methods of the present invention. Chemical mutagens include, for example, sodium bisulfite, nitrous acid, nitrosoguanidine, hydroxylamine, agents which damage or remove bases thereby preventing normal base-pairing such as hydrazine or formic acid, analogues of nucleotide precursors such as 5-bromouracil, 2-aminopurine, or acridine intercalating agents such as proflavine, acriflavine, quinacrine, and the like. Generally, plasmid DNA or DNA fragments are treated with chemical mutagens, transformed into E. coli and propagated as a pool or library of mutant plasmids. In constructing a hybrid PKS of the invention, regions encoding enzymatic activity, i.e., regions encoding corresponding activities from different PKS synthases or from different locations in the same PKS, can be recovered, for example, using PCR techniques with appropriate primers. By "corresponding" activity encoding regions is meant those regions encoding the same general type of activity. For example, a KR activity encoded at one location of a gene cluster "corresponds" to a KR encoding activity in another location in the gene cluster or in a different gene cluster. Similarly, a complete reductase cycle could be considered corresponding. For example, KR/DFI ΕR can correspond to a KR alone. If replacement of a particular target region in a host PKS is to be made, this replacement can be conducted in vitro using suitable restriction enzymes. The replacement can also be effected in vivo using recombinant techniques involving homologous sequences framing the replacement gene in a donor plasmid and a receptor region in a recipient plasmid. Such systems, advantageously involving plasmids of differing temperature sensitivities are described, for example, in PCT publication No. WO 96/40968, incorporated herein by reference. The vectors used to perform the various operations to replace the enzymatic activity in the host PKS genes or to support mutations in these regions of the host PKS genes can be chosen to contain control sequences operably linked to the resulting coding sequences in a manner such that expression of the coding sequences can be effected in an appropriate host. However, simple cloning vectors may be used as well. If the cloning vectors employed to obtain PKS genes encoding derived PKS lack control sequences for expression operably linked to the encoding nucleotide sequences, the nucleotide sequences are inserted into appropriate expression vectors. This need not be done individually, but a pool of isolated encoding nucleotide sequences can be inserted into expression vectors, the resulting vectors transformed or transfected into host cells, and the resulting cells plated out into individual colonies. The invention provides a variety of recombinant DNA compounds in which the various coding sequences for the domains and modules of the megalomicin PKS are flanked by non-naturally occurring restriction enzyme recognition sites. The various PKS nucleotide sequences can be cloned into one or more recombinant vectors as individual cassettes, with separate control elements, or under the control of, e.g., a single promoter. The PKS subunit encoding regions can include flanking restriction sites to allow for the easy deletion and insertion of other PKS subunit encoding sequences so that hybrid PKSs can be generated. The design of such unique restriction sites is known to those of skill in the art and can be accomplished using the techniques described above, such as site-directed mutagenesis and PCR. The expression vectors containing nucleotide sequences encoding a variety of PKS enzymes for the production of different polyketides are then transformed into the appropriate host cells to construct the library. In one straightforward approach, a mixture of such vectors is transformed into the selected host cells and the resulting cells plated into individual colonies and selected to identify successful transformants. Each individual colony has the ability to produce a particular PKS synthase and ultimately a particular polyketide. Typically, there will be duplications in some, most, or all of the colonies; the subset of the transformed colonies that contains a different PKS in each member colony can be considered the library. Alternatively, the expression vectors can be used individually to transform hosts, which transformed hosts are then assembled into a library. A variety of strategies are available to obtain a multiplicity of colonies each containing a PKS gene cluster derived from the naturally occurring host gene cluster so that each colony in the library produces a different PKS and ultimately a different polyketide. The number of different polyketides that are produced by the library is typically at least four, more typically at least ten, and preferably at least 20, and more preferably at least 50, reflecting similar numbers of different altered PKS gene clusters and PKS gene products. The number of members in the library is arbitrarily chosen; however, the degrees of freedom outlined above with respect to the variation of starter, extender units, stereochemistry, oxidation state, and chain length enables the production of quite large libraries. Methods for introducing the recombinant vectors of the invention into suitable hosts are known to those of skill in the art and typically include the use of CaCl 2 or agents such as other divalent cations, lipofection, DMSO, protoplast transformation, conjugation, infection, transfection, and electroporation. The polyketide producing colonies can be identified and isolated using known techniques and the produced polyketides further characterized. The polyketides produced by these colonies can be used collectively in a panel to represent a library or may be assessed individually for activity. The libraries of the invention can thus be considered at four levels: (1) a multiplicity of colonies each with a different PKS encoding sequence; (2) the proteins produced from the coding sequences; (3) the polyketides produced from the proteins assembled into a functional PKS; and (4) antibiotics or compounds with other desired activities derived from the polyketides. Of course, combination libraries can also be constructed wherein members of a library derived, for example, from the megalomicin PKS can be considered as a part of the same library as those derived from, for example, the rapamycin PKS or DEBS. Colonies in the library are induced to produce the relevant synthases and thus to produce the relevant polyketides to obtain a library of polyketides. The polyketides secreted into the media can be screened for binding to desired targets, such as receptors, signaling proteins, and the like. The supernatants per se can be used for screening, or partial or complete purification of the polyketides can first be effected. Typically, such screening methods involve detecting the binding of each member of the library to receptor or other target ligand. Binding can be detected either directly or through a competition assay. Means to screen such libraries for binding are well known in the art and can be applied in accordance with the methods of the present invention. Alternatively, individual polyketide members of the library can be tested against a desired target. In this event, screens wherein the biological response of the target is measured can more readily be included. Antibiotic activity can be verified using typical screening assays such as those set forth in Lehrer et al, 1991, J. Immunol. Meth. 737:167-173, incorporated herein by reference, and in the Examples below. The invention provides methods for the preparation of a large number of polyketides. These polyketides are useful intermediates in formation of compounds with antibiotic or other activity through hydroxylation, epoxidation, and glycosylation reactions as described above. In general, the polyketide products of the PKS must be further modified, typically by hydroxylation and glycosylation, to exhibit potent antibiotic activity. Hydroxylation results in the novel polyketides of the invention that contain hydroxyl groups at C-6, which can be accomplished using the hydroxylase encoded by the eryF gene, and/or C-12, which can be accomplished using the hydroxylase encoded by the picK or eryK gene. Also, the oleP gene is available in recombinant form, which can be used to express the oleP gene product in any host cell. A host cell, such as a Streptomyces host cell or a Saccharopolyspora erythraea host cell, modified to express the oleP gene thus can be used to produce polyketides comprising the C-8-C-8a epoxide present in oleandomycin. Thus the invention provides such modified polyketides. The presence of hydroxyl groups at these positions can enhance the antibiotic activity of the resulting compound relative to its unhydroxylated counterpart. Methods for glycosylating polyketides are generally known in the art and can be applied in accordance with the methods of the present invention; the glycosylation may be effected intracellularly by providing the appropriate glycosylation enzymes or may be effected in vitro using chemical synthetic means as described herein and in PCT publication No. WO 98/49315, incorporated herein by reference. Preferably, glycosylation with desosamine, mycarose, and/or megosamine is effected in accordance with the methods of the invention in recombinant host cells provided by the invention. In general, the approaches to effecting glycosylation mirror those described above with respect to hydroxylation. The purified enzymes, isolated from native sources or recombinantly produced may be used in vitro. Alternatively and as noted, glycosylation may be effected intracellularly using endogenous or recombinantly produced intracellular glycosylases. In addition, synthetic chemical methods may be employed. The antibiotic modular polyketides may contain any of a number of different sugars, although D-desosamine, or a close analog thereof, is most common. Erythromycin, picromycin, megalomicin, narbomycin, and methymycin contain desosamine. Erythromycin also contains L-cladinose (3-O-methyl mycarose). Tylosin contains mycaminose (4-hydroxy desosamine), mycarose and 6-deoxy-D-allose. 2-acetyl-l-bromodesosamine has been used as a donor to glycosylate polyketides by Masamune et al, 1975, J. Am. Chem. Soc. 97: 3512- 3513. Other, apparently more stable donors include glycosyl fluorides, thioglycosides, and trichloroacetimidates; see Woodward et al, 1981, J. Am. Chem. Soc. 103: 3215; Martin et al, 1997, J. Am. Chem. Soc. 119: 3193; Toshima et al, 1995, J. Am. Chem. Soc. 117: 3717; Matsumoto et al, 1988, Tetrahedron Lett. 29: 3575. Glycosylation can also be effected using the polyketide aglycones as starting materials and using Saccharopolyspora erythraea or Streptomyces venezuelae or other host cell to make the conversion, preferably using mutants unable to synthesize macrolides, as discussed in the preceding Section. Thus, a wide variety of polyketides can be produced by the hybrid PKS enzymes of the invention. These polyketides are useful as antibiotics and as intermediates in the synthesis of other useful compounds, as described in the following section. Section VII: Host Cells Containing Multiple Expression Vectors A recombinant host cell of the invention may contain nucleic acid encoding a megalomicin PKS domain, module, or protein, or megalomicin modification enzyme at a single genetic locus, e.g., on a single plasmid or at a single chromosomal locus, or at different genetic loci, e.g., on separate plasmids and/or chromosomal loci. By "multiple" is meant two or more; by "vector" is meant a nucleic acid molecule which can be used to transform host systems and which contains an independent expression system containing a coding sequence under control of a promoter and optionally a selectable marker and any other suitable sequences regulating expression. Typical such vectors are plasmids, but other vectors such as phagemids, cosmids, viral vectors and the like can be used according to the nature of the host. Of course, one or more of the separate vectors may integrate into the chromosome of the host (selection may not be required for maintenance of integrated vectors). In one embodiment, the invention provides a recombinant host cell, which comprises at least two separate autonomously replicating recombinant DNA expression vectors, each of said vectors comprises a recombinant DNA compound encoding a megalomicin PKS domain or a megalomicin modification enzyme operably linked to a promoter. In another embodiment, the invention provides a recombinant host cell, which comprises at least one autonomously replicating recombinant DNA expression vector and at least one modified chromosome, each of said vector(s) and each of said modified chromosome comprises a recombinant DNA compound encoding a megalomicin PKS domain or a megalomicin modification enzyme operably linked to a promoter. Preferably, the autonomously replicating recombinant DNA expression vector and/or the modified chromosome further comprises distinct selectable markers. The above multiple-vector (chromosome) expression systems can also be used for expressing heterogeneous polyketide biosynthetic enzymes, e.g., for expressing Micromonospora megalomicea megalomicin PKS protein, module, or domain or a megalomicin modification enzyme with a PKS protein, module, or domain, or modification enzyme from other origins in the same host cells. By placing various activities on different expression vectors, a high degree of variation can be achieved in an efficient manner. A variety of hosts can be used; any suitable host cell that can maintain multiple vectors can readily be used. Preferred hosts include Streptomyces, yeast, E. coli, other actinomycetes, and plant cells, and mammalian or insect cells or other suitable recombinant hosts can also be used. Preferred among yeast strains are Saccharomyces cerevisiae and Pichia pastoris. Preferred actinomycetes include various strains of Streptomyces. If one chooses to use a host cell that does not naturally produce a polyketide, then one may need to ensure that the recombinant host is modified to also contain a holo ACP synthase activity that effects pantetheinylation of the acyl carrier protein. See PCT Pub. No. WO 97/13845, incorporated herein by reference. One of the multiple vectors may be used for this purpose. This activation step is necessary for activation of the ACP. The expression system for the holo ACP synthase may be supplied on a vector separate from that carrying a PKS coding sequence or may be supplied on the same vector or may be integrated into the chromosome of the host, or may be supplied as an expression system for a fusion protein with all or a portion of a polyketide synthase (see U.S. Patent No. 6,033,883, incorporated herein by reference). It should be noted that in some recombinant hosts, it may also be necessary to activate the polyketides produced through postsynthesis modifications when polyketides having such modifications are desired. If this is the case for a particular host, the host will be modified, for example by transformation, to contain those enzymes necessary for effecting these modifications. Among such enzymes, for example, are glycosylation enzymes. The use of multiple vectors can facilitate the introduction of expression systems for such enzymes. In a preferred embodiment, the multiple vector system is used to assemble rapidly and efficiently a combinatorial library of polyketides and the PKS/modification enzymes that produce them. In an illustrative embodiment, the multiple vector system comprises four different vectors, one comprising the megAI gene, one the megAIl gene, one the megAIll gene, and one the modification enzyme(s) gene(s). Each of these vectors can be modified to make a set of vectors. For example, one set could contain all possible AT substitutions in the loading and first and second extender modules of the megAI gene product. Another set could contain expression systems for a variety of different modification enzymes. With these four vectors sets and by combining each member of each set with each member of the other three sets, a very large library of cells, vector sets, and polyketides can be rapidly and efficiently assembled. The combinatorial potential of a modular PKS such as the megalomicin PKS (ignoring the additional potential of different modification enzyme systems) is minimally given by: AT.. X (ATg X 4) M where AT is the number of loading acyl transferases, AT R is the number of extender acyl transferases, and M is the number of modules in the gene cluster. The number 4 is present in the formula because this represents the number of ways a keto group can be modified by either 1) no reaction; 2) KR activity alone; 3) KR+DH activity; or 4) KR+DH+ER activity. It has been shown that expression of only the first two modules of the erythromycin PKS resulted in the production of a predicted truncated triketide product (See Kao et al., J Am. Chem. Soc, 1 16: 1 1612-1 1613 ((1994)). A novel 12-membered macrolide similar to methymycin aglycone was produced by expression of modules 1-5 of this PKS in S. coelicolor (See Kao et al., J. Am. Chem. Soc, U7:9105-9106 (1995)). This work shows that PKS modules are functionally independent so that lactone ring size can be controlled by the number of modules present. In addition to controlling the number of modules, the modules can be genetically modified, for example, by the deletion of a ketoreductase domain as described by Donadio et al., Science, 252:675-679 (1991); and Donadio et al., Gene, 1 15:97-103 (1992). In addition, the mutation of an enoyl reductase domain was reported by Donadio, et al., Proc. Natl. Acad. Sci., 90:71 19-7123 (1993). These modifications also resulted in modified PKS and thus modified polyketides. As stated above, in the present invention, the coding sequences for catalytic activities derived from the megalomicin PKS systems found in nature can be used in their native forms or modified by standard mutagenesis techniques to delete or diminish activity or to introduce an activity into a module in which it was not originally present. For example, a KR activity can be introduced into a module normally lacking that function. In one embodiment of the invention herein, a single host cell is modified to contain a multiplicity of vectors, each vector contributing a portion of the synthesis of a megalomicin PKS and modification enzyme (if any) system. Each of the multiple vectors for production of the megalomicin PKS system typically encodes at least two modules, and at least one of the vectors integrates into the chromosome of the host. Integration can be effected using suitable phage or integrating vectors or by homologous recombination. If homologous recombination is used, the integration event may also be designed to delete endogenous PKS genes residing in the chromosome, as described in the PCT application WO 95/08548. In these embodiments, too, a selectable marker such as hygromycin or thiostrepton resistance can be included in the vector that effects integration. As mentioned above, additional enzymes that effect post-translational modifications to the enzyme systems in the megalomicin PKS may be introduced into the host through suitable recombinant expression systems. In addition, enzymes that activate the polyketides themselves, for example, through glycosylation may be added. It may also be desirable to modify the cell to produce more of a particular substrate utilized in polyketide biosynthesis. For example, it is generally believed that malonyl CoA levels in yeast are higher than methylmalonyl CoA; if yeast is chosen as a host, it may be desirable to increase methylmalonyl CoA levels by the addition of one or more biosynthetic enzymes therefor. The multiple-vector expression system can also be used to make polyketides produced by the addition of synthetic starter units to a PKS that contains an inactivated ketosynthase (KS) in the first module. As noted above, this modification permits the system to incorporate a suitable diketide thioester such as 3-hydroxy-2-methyl pantonoic acid-N-acetyl cysteamine thioester, or similar thioesters of diketide analogs, as described by Jacobsen et al., Science, 277:367-369 (1997). The construction of PKS modules containing inactivated ketosynthase regions can be conducted by methods known in the art, such as the method described in U.S. Patent No. 6,080,555 and PCT publication Nos. WO 99/03986 and 97/02358, each of which is incorporated herein by reference, in accordance with the methods of the present invention. The multiple-vector expression system can be used to produce polyketides in hosts that normally do not produce them, such as E. coli and yeast. It also provides more efficient means to provide a variety of polyketide products by supplying the elements of the introduced PKS, whether in an E. coli or yeast host or in other more traditionally used hosts, such as Streptomyces. The invention also includes libraries of polyketides prepared using the methods of the invention. Section VIII: Compounds The methods and recombinant DNA compounds of the invention are useful in the production of polyketides. In one important aspect, the invention provides methods for making antibiotic compounds related in structure to erythromycin, a potent antibiotic compound. The invention also provides novel ketolide compounds, polyketide compounds with potent antibiotic activity of significant interest due to activity against antibiotic resistant strains of bacteria. See Griesgraber et al, 1996, J. Antibiot. 49: 465-477, incorporated herein by reference. Most if not all of the ketolides prepared to date are synthesized using erythromycin A, a derivative of 6-dΕB, as an intermediate. In one embodiment, the present invention provides the 3-keto derivatives of the megalomicins for use as antibiotics. In particular, the 3-keto derivative of megalomicin A is a preferred ketolide of the invention. These compounds can be made chemically, substantially in accordance with the procedures for making ketolides described in the prior art, or in recombinant host cells of the invention in which the megosamine and desosamine biosynthetic and transferase genes are present but which do not make or transfer the mycarose moiety and/or the PKS has been modified to delete the KR domain of extender module 6. The invention also provides methods for making intermediates useful in preparing traditional, 6-dEB- and erythromycin- derived ketolide compounds. See 'Griesgraber et al, supra; Agouridas et al., 1998, J. Med. Chem. 41: 4080-4100, U.S. Patent Nos. 5,770,579; 5,760,233; 5,750,510; 5,747,467; 5,747,466; 5,656,607; 5,635,485; 5,614,614; 5,556,118; 5,543,400; 5,527,780; 5,444,051; 5,439,890; 5,439,889; and PCT publication Nos. WO 98/09978 and 98/28316, each of which is incorporated herein by reference. As noted above, the hybrid PKS genes of the invention can be expressed in a host cell that contains the desosamine, megosamine, and/or mycarose biosynthetic genes and corresponding transferase genes as well as the required hydroxylase gene(s), which may, for example and without limitation, be either picK, megK, or eryK (for the C-12 position) and/or megF oτeryF (for the C-6 position). The resulting compounds have antibiotic activity but can be further modified, as described in the patent publications referenced above, to yield a desired compound with improved or otherwise desired properties. Alternatively, the aglycone compounds can be produced in the recombinant host cell, and the desired glycosylation and hydroxylation steps carried out in vitro or in vivo, in the latter case by supplying the converting cell with the aglycone, as described above. The compounds of the invention are thus optionally glycosylated forms of the polyketide set forth in formula (1) below which are hydroxylated at either the C-6 or the C-12 or both. The compounds of formula (1) can be prepared using the loading and the six extender modules of a modular PKS, modified or prepared in hybrid form as herein described. These polyketides have the formula: including the glycosylated and isolated stereoisomeric forms thereof; wherein R* is a straight chain, branched or cyclic, saturated or unsaturated substituted or unsubstituted hydrocarbyl of 1 -15C; each of R'-R 6 is independently H or alkyl (1-4C) wherein any alkyl at R 1 may optionally be substituted; each of X'-X 5 is independently two H, H and OH, or =O; or each of X'-X 5 is independently H and the compound of formula (2) contains a double-bond in the ring adjacent to the position of said X at 2-3, 4-5, 6- 7, 8-9 and/or 10-1 1; with the proviso that: at least two of R'-R 6 are alkyl (1-4C). Preferred compounds comprising formula 2 are those wherein at least three of R'-R 3 are alkyl (1-4C), preferably methyl or ethyl; more preferably wherein at least four of R'-R 5 are alkyl (1-4C), preferably methyl or ethyl. Also preferred are those wherein X 2 is two H, =O, or H and OH, and/or X 3 is H, and/or X 1 is OH and/or X 4 is OH and/or X 5 is OH. Also preferred are compounds with variable R* when R'-R 5 is methyl, X 2 is =O, and X 1 , X 4 and X 5 are OH. The glycosylated forms (i.e., mycarose or cladinose at C-3, desosamine at C-5, and/or megosamine at C-6) of the foregoing are also preferred. As described above, there are a wide variety of diverse organisms that can modify compounds such as those described herein to provide compounds with or that can be readily modified to have useful activities. For example, Saccharopolyspora erythraea can convert 6-dEB to a variety of useful compounds. The compounds provided by the present invention can be provided to cultures of Saccharopolyspora erythraea and converted to the corresponding derivatives of erythromycins A, B, C, and D in accordance with the procedure provided in the Examples, below. To ensure that only the desired compound is produced, one can use an S. erythraea eryA mutant that is unable to produce 6- dEB but can still carry out the desired conversions (Weber et al, 1985, J. Bacteriol. 164(\): 425-433). Also, one can employ other mutant strains, such as eryB , eryC, eryG, and/or eryK mutants, or mutant strains having mutations in multiple genes, to accumulate a preferred compound. The conversion can also be carried out in large fermentors for commercial production. Each of the erythromycins A, B, C, and D has antibiotic activity, although erythromycin A has the highest antibiotic activity. Moreover, each of these compounds can form, under treatment with mild acid, a C-6 to C-9 hemiketal with motilide activity. For formation of hemiketals with motilide activity, erythromycins B, C, and D, are preferred, as the presence of a C-12 hydroxyl allows the formation of an inactive compound that has a hemiketal formed between C-9 and C-12. Thus, the present invention provides the compounds produced by hydroxylation and glycosylation of the compounds of the invention by action of the enzymes endogenous to Saccharopolyspora erythraea and mutant strains of S. erythraea. Such compounds are useful as antibiotics or as motilides directly or after chemical modification. For use as antibiotics, the compounds of the invention can be used directly without further chemical modification. Erythromycins A, B, C, and D all have antibiotic activity, and the corresponding compounds of the invention that result from the compounds being modified by Saccharopolyspora erythraea also have antibiotic activity. These compounds can be chemically modified, however, to provide other compounds of the invention with potent antibiotic activity. For example, alkylation of erythromycin at the C-6 hydroxyl can be used to produce potent antibiotics (clarithromycin is C-6-O- methyl), and other useful modifications are described in, for example, Griesgraber et al, 1996, J. Antibiot. 49: 465-477, Agouridas et al, 1998, J. Med. Chem. 41: 4080-4100, U.S. Patent Nos. 5,770,579; 5,760,233; 5,750,510; 5,747,467; 5,747,466; 5,656,607; 5,635,485; 5,614,614; 5,556,118; 5,543,400; 5,527,780; 5,444,051 ; 5,439,890; and 5,439,889; and PCT publication Nos. WO 98/09978 and 98/28316, each of which is incorporated herein by reference. For use as motilides, the compounds of the invention can be used directly without further chemical modification. Erythromycin and certain erythromycin analogs are potent agonists of the motilin receptor that can be used clinically as prokinetic agents to induce phase III of migrating motor complexes, to increase esophageal peristalsis and LES pressure in patients with GERD, to accelerate gastric emptying in patients with gastric paresis, and to stimulate gall bladder contractions in patients after gallstone removal and in diabetics with autonomic neuropathy. See Peeters, 1999, Motilide Web Site, http://www.med. kuleuven. ac.be/med/gih motilid.htm, and Omura et al., 1987, Macrolides with gastrointestinal motor stimulating activity, J. Med. Chem. 30: 1941-3). The corresponding compounds of the invention that result from the compounds of the invention being modified by Saccharopolyspora erythraea also have motilide activity, particularly after conversion, which can also occur in vivo, to the C-6 to C-9 hemiketal by treatment with mild acid. Compounds lacking the C-12 hydroxyl are especially preferred for use as motilin agonists. These compounds can also be further chemically modified, however, to provide other compounds of the invention with potent motilide activity. Moreover, and also as noted above, there are other useful organisms that can be employed to hydroxylate and/or glycosylate the compounds of the invention. As described above, the organisms can be mutants unable to produce the polyketide normally produced in that organism, the fermentation can be carried out on plates or in large fermentors, and the compounds produced can be chemically altered after fermentation. In addition to Saccharopolyspora erythraea, Streptomyces venezuelae, S. narbonensis, S. antibioticus, Micromonospora megalomicea, S. fradiae, and S. thermotolerans can also be used. In addition to antibiotic activity, compounds of the invention produced by treatment with M. megalomicea enzymes can have antiparasitic activity as well. Thus, the present invention provides the compounds produced by hydroxylation and glycosylation by action of the enzymes endogenous to S. erythraea, S. venezuelae, S. narbonensis, S. antibioticus, M. megalomicea, S. fradiae, and S. thermotolerans. The present invention also provides methods and genetic constructs for producing the glycosylated and/or hydroxylated compounds of the invention directly in the host cell of interest. Thus, the recombinant genes of the invention, which include recombinant megAI, megAIl, and megAIll genes with one or more deletions and/or insertions, including replacements of a megA gene fragment with a gene fragment from a heterologous PKS gene, can be included on expression vectors suitable for expression of the encoded gene products in Saccharopolyspora erythraea, Micromonospora megalomicea, S. venezuelae, S. narbonensis, S. antibioticus, S. fradiae, and S. thermotolerans. The compounds of the invention can be produced by growing and fermenting the host cells of the invention under conditions known in the art for the production of other polyketides. The compounds of the invention can be isolated from the fermentation broths of these cultured cells and purified by standard procedures. The compounds can be readily formulated to provide the pharmaceutical compositions of the invention. The pharmaceutical compositions of the invention can be used in the form of a pharmaceutical preparation, for example, in solid, semisolid, or liquid form. This preparation will contain one or more of the compounds of the invention as an active ingredient in admixture with an organic or inorganic carrier or excipient suitable for external, enteral, or parenteral application. The active ingredient may be compounded, for example, with the usual non-toxic, pharmaceutically acceptable carriers for tablets, pellets, capsules, suppositories, solutions, emulsions, suspensions, and any other form suitable for use. The carriers which can be used include water, glucose, lactose, gum acacia, gelatin, mannitol, starch paste, magnesium trisilicate, talc, corn starch, keratin, colloidal silica, potato starch, urea, and other carriers suitable for use in manufacturing preparations, in solid, semi-solid, or liquified form. In addition, auxiliary stabilizing, thickening, and coloring agents and perfumes may be used. For example, the compounds of the invention may be utilized with hydroxypropyl methylcellulose essentially as described in U.S. Patent No. 4,916,138, incorporated herein by reference, or with a surfactant essentially as described in EPO patent publication No. 428,169, incorporated herein by reference. Oral dosage forms may be prepared essentially as described by Hondo et al, 1987, Transplantation Proceedings XIX, Supp. 6: 17-22, incorporated herein by reference. Dosage forms for external application may be prepared essentially as described in EPO patent publication No. 423,714, incorporated herein by reference. The active compound is included in the pharmaceutical composition in an amount sufficient to produce the desired effect upon the disease process or condition. For the treatment of conditions and diseases caused by infection, a compound of the invention may be administered orally, topically, parenterally, by inhalation spray, or rectally in dosage unit formulations containing conventional non-toxic pharmaceutically acceptable carriers, adjuvant, and vehicles. The term parenteral, as used herein, includes subcutaneous injections, and intravenous, intramuscular, and intrasternal injection or infusion techniques. Dosage levels of the compounds of the invention are of the order from about 0.01 mg to about 50 mg per kilogram of body weight per day, preferably from about 0.1 mg to about 10 mg per kilogram of body weight per day. The dosage levels are useful in the treatment of the above-indicated conditions (from about 0.7 mg to about 3.5 mg per patient per day, assuming a 70 kg patient). In addition, the compounds of the invention may be administered on an intermittent basis, i.e., at semi-weekly, weekly, semi-monthly, or monthly intervals. The amount of active ingredient that may be combined with the carrier materials to produce a single dosage form will vary depending upon the host treated and the particular mode of administration. For example, a formulation intended for oral administration to humans may contain from 0.5 mg to 5 gm of active agent compounded with an appropriate and convenient amount of carrier material, which may vary from about 5 percent to about 95 percent of the total composition. Dosage unit forms will generally contain from about 0.5 mg to about 500 mg of active ingredient. For external administration, the compounds of the invention may be formulated within the range of, for example, 0.00001% to 60%) by weight, preferably from 0.001%) to 10% by weight, and most preferably from about 0.005% to 0.8%) by weight. It will be understood, however, that the specific dose level for any particular patient will depend on a variety of factors. These factors include the activity of the specific compound employed; the age, body weight, general health, sex, and diet of the subject; the time and route of administration and the rate of excretion of the drug; whether a drug combination is employed in the treatment; and the severity of the particular disease or condition for which therapy is sought. A detailed description of the invention having been provided above, the following examples are given for the purpose of illustrating the invention and shall not be construed as being a limitation on the scope of the invention or claims. Example 1 Cloning and Characterization of the Megalomicin Biosynthetic Gene Cluster from Micromonospora meglomicea Experimental Procedures Bacterial Strains, Media, and Growth Conditions Routine DNA manipulations were performed in Escherichia coli XL 1 Blue or E. coli XLl Blue MR (Stratagene) using standard culture conditions (Sambrook et al, 1989). M. megalomicea subs, nigra NRRL3275 was obtained from the ATCC collection and cultured according to recommended protocols. For isolation of genomic DNA, M. megalomicea was grown in TSB (Flopwood et al, 1985) at 30 °C. S. lividans K4-1 14 (Ziermann and Betlach, 1999), which carries a deletion of the actinorhodin biosynthetic gene cluster, was used as the host for expression of the megAI- AIII genes. S. lividans strains were maintained on R5 agar at 30°C and grown in liquid YEME for preparation of protoplasts (Hopwood et al, 1985) . S. erythraea NRRL2338 was used for expression of the megosamine genes. S. erythraea strains were maintained on R5 agar at 34°C and grown in liquid TSB for preparation of protoplasts. Manipulation of DNA and Organisms Manipulation and transformation of DNA in E. coli was performed by standard procedures (Sambrook et al, 1989) or by suppliers protocols. Protoplasts of S. lividans and S. erythraea were generated for transformation by plasmid DNA using the standard procedure. S. lividans transformants were selected on R5 using 2 ml of a 0.5 mg/ml thiostrepton overlay. S. erythraea transformants were selected on R5 using 1.5 ml of a 0.6 mg/ml apramycin overlay. Isolation of the meg gene cluster A cosmid library was prepared in SuperCos (Stratagene) from M. megalomicea total DNA partially digested with Sau3A I, and introduced into E. coli using a Gigapack 111 XL (Stratagene) in-vitro packaging kit. P-labelled DNA probes encompassing the KS2 domain from ery DΕBS, or a mixture of segments encompassing modules 1 and 2 from ery DΕBS were used separately to screen the cosmid library by colony hybridization. Several colonies which hybridized with the probes were further analyzed by sequencing the ends of their cosmid inserts using T3 and T7 primers. BLAST (Altschul et al, 1990) analysis of the sequences revealed several colonies with DNA sequences highly homologous to genes from the ery cluster. Together with restriction analysis, this led to the isolation of two overlapping cosmids, pKOS079-93A and pKOS079-93D which covered -45 kb of the meg cluster. A 400 bp PCR fragment was generated from the left end of and pKOS079-93D and used to reprobe the cosmid library. Likewise, a 200 bp PCR fragment generated from the right end of pKOS079-93A was used to reprobe the cosmid library. Analysis of hybridizing colonies as described above resulted in identification of two additional cosmids, pKOS079-138B and pKOS79-124B which overlap the previous two cosmids. BLAST analysis of the far left and right end sequences of these cosmids indicated no homology to any known genes related to polyketide biosynthesis and therefore indicates that the set of four cosmids spans the entire megalomicin biosynthetic gene cluster. DNA sequencing and analysis PCR-based double stranded DNA sequencing was performed on a Beckman CΕQ 2000 capillary sequencer using reagents and protocols provided by the manufacturer. A shotgun library of the entire cosmid pKOS079-93D insert was made as follows: DNA was first digested with Dra 1 to eliminate the vector fragment, then partially digested with Sau3A I. After agarose electrophoresis, bands between 1 -3 kb were excised from the gel and ligated with BamH I digested pUC19. Another shotgun library was generated from a 12 kb Xho VEcoR. I fragment subcloned from cosmid pKOS079-93A to extend the sequence to the megF gene. A 4 kb Bgl 11/ Xho I fragment from cosmid pKOS079-138B was sequenced by primer walking to extend the sequencing to the megT gene. Sequence was assembled using Sequencher (Gene Codes Corp.) software package and analyzed with MacVector (Oxford Molecular Group) and the NCB1 BLAST server (www.ncbi.nlm.nih.gov/BLAST/). Plasmids Plasmid pKOS108-6 is a modified version of pKAO127'kan' (Ziermann and Betlach, 1999; Ziermann and Betlach, 2000) in which the eryAI-III genes between the Pac I and EcoR I sites have been replaced with the megp \-W\ genes. This was done by first substituting a synthetic nucleotide DNA duplex (5'- TAAGAATTCGGAGATCTGGCCTCAGCTCTAGAC (SΕQ ID NO: 21), complementary oligo 5'- AATTGTCTAGAGCTGAGGCCAGATCTCCGAATTCTTAAT (SΕQ ID NO: 22)) between the Pac I and EcoR I sites of the pKAO127'kan' vector fragment. The 22 kb EcoR llBgl II fragment from cosmid pKOS079-93D containing the megAI-II genes was inserted into EcoR I and Bgl II sites of the resulting plasmid to generate pKOS024-84. A 12 kb Bgl WIBbvC I fragment containing the megAIll and part of the megCll gene was subcloned from pKOS079-93A and excised as a Bgl lllXba I fragment and ligated into the corresponding sites of pKOS024-84 to yield the final expression plasmid pKOS 108-06. The megosamine integrating vector, pKOS97-42, was constructed as follows: A subclone was generated containing the 4 kb Xho l/Sca I fragment from pKOS79-138B together with the 1.7 kb Sea l/Pst I fragment from pKOS79-93D in Litmus 28 (Stratagene). The entire 5.7 kb fragment was then excised as a Spe VPst I fragment and combined with the 6.3 kb Pst I/EcoR I fragment from KOS79-93D and EcoR l/Xba I digested pSΕT152 (Bierman et al, 1992) to construct plasmid pKOS97-42. Production and analysis of secondary metabolites Fermentation for production of polyketide, LC/MS analysis, and quantification of 6-dEB for S. lividans K4-114/pKOS 108-6 and S. lividans K4- 1 14/pKAO127'kan' were essentially as previously described (Xue et al, 1999). S. erythraea NRRL2338 and S. eryt 7rαeα/pKOS97-42 were grown for 6 days in FI media (Brϋnker et al, 1998). Samples of broth were clarified in a microcentrifuge (5 min, 13,000 rpm). For LC/MS preparation, isopropanol was added to the supernatant (1 :2 ratio) and centrifuged again. Erythromycins and megalomicins were detected by electrospray mass spectrometry and quantity was determined by evaporative light scattering detection (ELSD). The LC retention time and mass spectra of erythromycin and megalomicins were identical to known standards. Nucleotide sequence of the meg gene cluster A series of 4 overlapping inserts containing the meg cluster (Figure 9) were isolated from a cosmid library prepared from total genomic DNA of M. megalomicea and covers > 100 kb of the genome. A contiguous 48 kb segment which encodes the megalomicin PKS and several deoxysugar biosynthetic genes was sequenced and analyzed. The segment contains 17 complete ORFs as well as an incomplete ORF at each end, organized as shown in Figure 9. PKS genes. The ORFs megAI, megAIl and megAIll encode the polyketide synthase responsible for synthesis of 6-dEB. The enzyme complex, meg DEBS, is highly similar to ery DEBS, with each of the three predicted polypeptides sharing an average of 83% overall similarity with their ery PKS counterpart. Both PKSs are composed of 6 modules (2 modules per polypeptide) and each module is organized in the identical manner (Figure 9). A dendrogram analysis (Schwecke et al, 1995) employing 70 acyltranferase (AT) domains revealed that the 6 meg extender AT domains cluster with AT domains that incorporate methylmalonyl CoA (not shown). The loading module of meg DEBS also lacks a KS Q domain which is utilized by most macrolide PKSs for decarboxylation of the starter unit to initiate polyketide synthesis (Bisang et al. , 1999; Kuhstoss et al. , 1996; Kakavas et al, 1997; Xue et al, 1998), implying that priming begins with a propionate unit. In addition, a conserved Gly to Pro substitution in the NADPH-binding region of the ketoreductase (KR) domain of module 3 is observed in meg DEBS, which has been proposed to account for its inactivity in ery DEBS (Donadio et al, 1991). Deoxysugar genes. BLAST (Altschul et al, 1990) analysis of the genes flanking the PKS indicated that 12 complete ORFs and 1 partial ORF appear to encode functions required for synthesis of one of the three megalomicin deoxysugars. Assignment of each ORF to a specific deoxysugar pathway was made based on comparison to the ery genes and other related genes involved in deoxysugar biosynthesis (Table 2). Table 2 Deduced functions of genes identified in the megalomicin gene cluster Gene Closest Match % Sim 3 Proposed Proposed Function Reference (polypeptide) 1 ' Pathway megT EryBVI Mycarose/ 2,3-Dehydratase (Summers et al , 1997, Megosamine Gaisser et al , 1997) megDV! EryCII 63 Megosamine 3,4-Isomerase (Summers et al , 1997) megDI EryCIII 79 Megosamine G lycosyltransferase (Summers et al , 1997) megY AcyA (S 52 Mycarose O-acyl- ( Aπsawa et al , 1994) thermotolerans) transfεrase megDI! EryCI 58 Megosamine Amtnotransferase (Dhillon et al , 1989, Summers et al , 1997) megDlll DesVl (S 61 Megosamine Dimethyltransferase (Xue et al , 1998) venezuelae) megDIV DmnU (S 65 Megosamine 3,5-Epιmerase (Olano . α/ , 1999) peucetius) megDV Dehydrogenase 61 Megosamine 4-Ketoreductase (Summers et al , 1997, van (A on en talis) Wageningen et al , 1998) megDV!! EryBII 73 Megosamine 2,3-Reductase (Summers et al , 1997) megBV EryBV 86 Mycarose G lycosyltransferase (Summers et al , 1997, Gaisser et al , 1997) megBIV EryBIV 80 Mycarose 4-Ketoreductase (Summers et al , 1997, Gaisser et al , 1997) megAI EryAI 81 6-dEB Polyketide Synthase (Donadio and Katz, 1992) megAIl EryAII 85 6-dEB Polyketide Synthase (Donadio and Katz, 1992) megAIll EryAIII 83 6-dEB Polyketide Synthase (Donadio and Katz, 1992) megCII EryCII 82 Desosamine 3,4-Isomerase (Summers et al , 1997) meg CHI EryCIII 89 Desosamine Glycosylyltransferase (Summers et al , 1997) megBII EryBII 87 Mycarose 2,3-Reductase (Summers et al , 1997) megH EryH 84 Thioesterase (Haydock et al , 1991) megF EryF C-6 Hydroxylase (Weber et at , 1991 ) a Determined by BLASTX analysts using default parameters Three ORFs, megBV, megCIII and megDI, encode glycosyltransferases, apparently one for attachment of each deoxysugar to the macrolide. MegBV was most similar to EryBV, the erythromycin mycarosyltransferase, and hence was assigned to the mycarose pathway in the meg cluster. The closest match for both of the remaining glycosyltransferases was EryCIII, the desosaminyltransferase in erythromycin biosynthesis. Given the higher degree of similarity between EryCIII and MegCIII (Table 2), MegCIII was designated the desosaminyltransferase, leaving MegDI as the proposed megosaminyltransferase. In similar fashion, assignments were made accordingly for; MegCII and MegDVI, two putative 3,4- isomerases similar to EryCII; MegBII and MegDVII, 2,3-reductases homologous to EryBII; MegBIV and MegDV, putative 4-ketoreductases similar to EryBIV (Table 2). The remaining ORFs involved in deoxysugar biosynthesis, megT, megDII, megDIII and megDIV, each encode a putative 2,3 -dehydratase, aminotransferase, dimethyltransferase and 3,5-epimerase, respectively (Table 2). Since both the megosamine and desosamine pathways require an aminotransferase and a dimethyltransferase, and since mycarose and megosamine each require a 2,3-dehydratase and a 3,5-epimerase, assignments of these four genes to a specific pathway could not be made on the basis of sequence comparison alone. However, the latter three are implicated in megosamine biosynthesis by experiments described below. Other genes. Two additional complete ORFs, designated megY and megH and an incomplete ORF, designated megF, were also identified in the cluster. MegH and MegF share high degrees of similarity with EryH and EryF. EryH and homologs in other macrolide gene clusters are thioesterase-like proteins with unknown function in polyketide gene clusters (Haydock et αl, 1991 ; Xue et αl, 1998; Butler et αl., 1999; Tang et αl., 1999). EryF encodes the erythronolide B C-6 hydroxylase (Figure 8) (Weber et αl, 1991; Andersen and Hutchinson, 1992). MegY does not have an ery counterpart but appears to belong to a (small) family of O-acyltransferases that transfer short acyl chains to macrolides. Two classes exist: AcyA and MdmB transfer acetyl or propionyl groups to the C-3 hydroxyls on 16-membered macrolide rings (Arisawa et αl., 1994; Hara and Hutchinson, 1992); CarE and Mpt transfer isovalerate or propionate to the mycarosyl moiety of carbomycin and midecamycin, respectively (Epp et αl, 1989; Arisawa et αl., 1993; Gu et al. , 1996). The structures of various megalomicins suggest that MegY belongs to the latter class and is the acyltransferase which converts megalomicin A to megalomicins B, Cl , or C2 (verified experimentally below). Heterologous expression of the meg PKS genes. The wild type and genetically modified versions of the ery DEBS have been used extensively in heterologous Streptomyces hosts for enzyme studies and the production of novel polyketide compounds. Given the similarities between the ery and meg DEBSs, production characteristics were compared in a commonly used Streptomyces host strain. The three megA ORFs were cloned into the expression plasmid pKA0127'kan' (Ziermann and Betlach, 1999) in place of the ery A ORFs. Both plasmids, pKAO127'kan' encoding ery DEBS and pKOS 108-06 encoding meg DEBS, were introduced in Streptomyces lividans K4-1 14 and the production of 6-dEB was determined in shake-flask fermentations. The production profiles were similar in both cases and the maximum titer of 6-dEB was between 30-40 mg/L. In addition, both PKSs produced small amounts (~5%>) of 8,8a- deoxyoleandolide, which results from the priming of the PKS with acetate instead of propionate (Kao et al, 1994b). This observation indicates that the loading AT domains of the PKSs display similar relaxed specificities towards starter units. Conversion of erythromycin to megalomicin in S. erythraea. An examination of the meg cluster revealed that the putative megosamine biosynthetic genes are clustered directly upstream of the PKS genes. If the hypothesis that these genes are sufficient for biosynthesis and attachment of megosamine to an erythromycin intermediate is correct, then functional expression of these genes in a strain which produces erythromycin, such as S. erythraea, should result in production of megalomicin. A 12 kb DNA fragment carrying all the genes between the leftmost _¥7zoI site and the EcoRI site (Figure 9) was integrated in the chromosome of S. erythraea using the site-specific integrating vector pSΕTl 52 (Bierman et al. , 1992). It was surmised that the left and right ends of this fragment would contain necessary promoter regions for transcription of the convergent set of genes in M. megalomicea and that they would likely operate in S. erythraea. Fermentation broth from S. erythraea/KOS97-42, which contains the integrated meg genes, was analyzed by LC/MS and compared to LC/MS profiles of the parent S. erythraea strain without the meg genes, as well as to megalomicin standards purified from M. megalomicea. The new strain was found to produce a mixture of erythromycin A and various megalomicins (~4: 1 ratio), thereby showing that the predicted megosamine biosynthetic and glycosyltransferase genes are contained within the cloned meg fragment. The two most abundant congeners identified were megalomicins B and Cl . Megalomicin A and C2 were also detected in smaller amounts. The presence of the megalomicins B, Cl and C2 also provides direct evidence for the function of the O-acyl transferase, MegY, which is present in the integrated meg fragment. Discussion The homologies observed among modular PKSs enabled the use of ery PKS genes to clone the meg biosynthetic gene cluster from M. megalomicea. The close similarities between the megalomicin and erythromycin biosynthetic pathways is also reflected in the overall organization of their genes and in the high degree of homology of the corresponding individual gene-encoded polypeptides. Production of 6-dEB from meg DEBS in S. lividans and conversion of erythromycin to megalomicin using the megD genes in S. erythraea provides direct evidence that the identified gene cluster is responsible for synthesis of megalomicin. As seen in Figure 9, the ~ 40 kb segments of the two clusters beginning with ery/megBV on the left through the ery/megF genes retain a nearly identical organizational arrangement. The notable differences in this region are eryG and IS773< which are absent from the segment of the meg cluster analyzed. The eryG gene encodes an S-adenosylmethionine (SAM)-dependent mycarosyl methyltransferase that converts erythromycin C to erythromycin A (Figure 8) (Weber et al, 1990; Haydock et al, 1991). The mycarose moiety is modified by esterification (MegY) in megalomicin biosynthesis (Figure 8) and, therefore, the absence of an eryG homolog would be expected in the meg cluster. The IS773<5 element located between eryAI and eryAII (Donadio and Staver, 1993) is not known to play a role in erythromycin biosynthesis and its origin in the ery cluster has not been determined. Upstream of the common meg/ ery BIV and BV genes, the gene clusters diverge. The ~ 6 kb segment between eryBV and eryK, the left border of the ery gene cluster (Pereda et αl., 1997), contains the remaining genes required for mycarose (eryBVI and BVIT) and desosamine biosynthesis (eryCIV, CV, and CVT) and the C-12 hydroxylase (eryK) (Stassi et αl., 1993). In contrast, the region upstream of megBV encodes a set of genes (megDI-DVII and megY) which can account for all the activities unique to megalomicin biosynthesis (Figure 9). Since introduction of this meg DNA segment into S. erythraea results in production of megalomicins, it is clear that these genes encode the functions for TDP- megosamine biosynthesis and transfer to its putative substrate erythromycin C, and to acylate megalomicin A (Figure 8). The remaining region upstream of megDVI should therefore encode genes only for mycarose and desosamine biosynthesis. Olano et al. (Olano et al. , 1999) have recently described a pathway for biosynthesis of TDP-L-daunosamine, a deoxysugar component of the antitumor compounds daunorubicin and doxorubicin produced by Streptomyces peucetius. Their pathway proposes four steps from the intermediate TDP-4-keto-6- deoxyglucose controlled by the gene cluster dnmJQTUVZ, although the functions for dnmQ and dnmZ could not be identified and the precise order of reactions in the pathway could not be determined. The genes dnmT, dnmU, dnmJ and dnmV each have proposed counterparts in the meg cluster, megT, megDIV, megDII, and megDV, respectively (see Figure 10) It is possible to describe a pathway to convert TDP-2,6-dideoxy-3,4- diketo-D-hexose (or its enol tautomer), the last intermediate common to the mycarose and megosamine pathways, to TDP-megosamine through the sequence of 5-epimerization, 4-ketoreduction, 3-amination, and 3-N-dimethylation employing the genes megDIV, megDV, megDII, and megDIII. This employs the same functions proposed for biosynthesis of TDP-daunosamine by Olano et al, but in a different sequential order. However, it does not account for the megDVI and megDVII genes since their activities are not required for this route. A parallel pathway which employs these genes is also shown in Figure 10. In this alternate route, 2,3-reduction and 3,4-tautomerization are performed by the megDVII and megDVI gene products, respectively. A unified single pathway that employs both 4-ketoreduction (megDV) and 2,3-reduction (megDVII) could not be determined. Because the entire gene set from megDVI through megDVII was introduced in S. erythraea to produce TDP-megosamine, it is not possible to determine which, if either, of the two alternative pathways is operative, but this can be addressed through systematic gene disruption and complementation. The 48 kb segment sequenced also contains genes required for synthesis of TDP-L-mycarose and TDP-D-desosamine (Fig 10). For the latter, megCII, which encodes a putative 3,4-isomerase, the first step in the committed TDP-desosamine pathway, appears to be translationally coupled to megAIll, almost exactly as its erythromycin counterpart, eryCII, was found translationally coupled to eryAIII (Summers et al, 1997). The high degree of similarity between MegCII and EryCII suggests that the pathway to desosamine in the megalomicin- and erythromycin- producing organisms are most likely the same. Similarly, the finding that megBII and megBIV, encoding a 2,3-reductase and 4-ketoreductase, contain close homologs in the mycarose pathway for erythromycin also suggests that TDP-L- mycarose synthesis in the two host organisms is the same. Of interest are the two genes that encode putative 2,3-reductases, megBII and megDVII. Because MegBII most closely resembles EryBII, a known mycarose biosynthetic enzyme (Weber et al. , 1990), and because megBII resides in the same location of the meg cluster as its counterpart in the ery cluster, megBII is assigned to the mycarose pathway and megDVII io the megosamine pathway. Furthermore, the lower degree of similarity between MegDVII and either EryBII or MegBII (Table 2) provides a basis for assigning the opposite L and D isomeric substrates to each of the enzymes (Figure 10). Finally, megT, which encodes a putative 2,3- dehydratase, is also related to a gene in the ery mycarose pathway, eryBVI. In S. erythraea, the proposed intermediate generated by EryBVI represents the first committed step in the biosynthesis of mycarose (Figure 10). However, the proposed pathways in Figure 10 suggest this may be an intermediate common to both mycarose and megosamine biosynthesis in M. megalomicea. Therefore, megT is named following the designation of the equivalent gene in the daunosamine pathway, dnmT (Olano et al, 1999) The preferred host-vector system for expression of meg DEBS described here has been used previously for the heterologous expression of modular PKS genes from the erythromycin (Kao et al, 1994a; Ziermann and Betlach, 1999), picromycin (Tang et al., 1999) and oleandomycin pathways, as well as for the generation of novel polyketide backbones where domains have been removed, added or exchanged in various combinations (McDaniel et al, 1999). Recently, hybrid polyketides have been generated through the co-expression of subunits from different PKS systems (Tang et al. , 2000). Expression of the megDVI-megDVII segment in S. erythraea and the corresponding production of megalomicins in this host establishes the likely order of sugar attachment in megalomicin synthesis. Furthermore, it provides a means to produce megalomicin in a more genetically friendly host organism, leading to the creation of megalomicin analogs by manipulating the PKS. Over 60 6-dEB analogs have been produced by combinatorial biosynthesis using the ery PKS (McDaniel et αl. , 1999; Xue et αl. , 1999). The titers of megalomicin could also be significantly increased above the 5 mg/L obtained from M. megαlomicieα by introducing the genes into an industrially optimized strain of S. erythraea, many of which can produce as much as 10 g/L of erythromycin. References Kao, CM., Katz, L. and Khosla, C. (1994a) Engineered biosynthesis of a complete macrolactone in a heterologous host. Science 265: 509-512. Kao, CM., Luo, G., Katz, L., Cane, D.E. and Khosla, C. (1994b) Engineered biosynthesis of a triketide lactone from an incomplete modular polyketide synthase. J. Am. Chem. Soc. 116: 1 1612-11613. McDaniel, R., Thamchaipenet, A., Gustafsson, C, Fu, H., Betlach, M., Betlach, M. et al (1999) Multiple genetic modifications of the erythromycin gene cluster to produce a library of novel "unnatural" natural products. Proc. Natl. Acad. Sci. USA 96: 1846-1851. Olano, C, Lomovskaya, N., Fonstein, L., Roll, J.T. and Hutchinson, C.R. (1999) A two-plasmid system for the glycosylation of polyketide antibiotics: bioconversion of e-rhodomycinone to rhodomycin D. Chem. & Biol 6: 845-855. Tang, L., Fu, H., Betlach, M.C and McDaniel, R. (1999) Elucidating the mechanism of chain termination switching in the picromycin/methymycin polyketide synthase. Chem. & Biol. 6: 553-558. Tang, L., Fu, H. and McDaniel, R. (2000) Formation of functional heterologous complexes using subunits from the picromycin, erythromycin, and oleandomycin polyketide synthases. Chem. & Biol. 7: 77-84. Weber, J.M., Leung, J.O., Maine, G.T., Potenz, R.H., Paulus, T.J. and DeWitt, J.P. (1990) Organization of a cluster of erythromycin genes in Saccharopolyspora erythraea. J. Bacteriol. 172: 2372-2383. Weber, J.M., Leung, J.O., Swanson, S.J., Idler, K.B. and McAlpine, J.B. (1991) An erythromycin derivative produced by targeted gene disruption in Saccharopolyspora erythraea. Science 252: 1 14-1 17. Xue, Q., Ashley, G., Hutchinson, C.R. and Santi, D.V. (1999) A multi-plasmid approach to preparing large libraries of polyketides. Proc. Natl. Acad. Sci. USA 96: 11740-1 1745. Xue, Y., Zhao, L., Liu, H.-w. and Sherman, D.H. (1998) A gene cluster for the macrolide antibiotic biosynthesis in Streptomyces venezuelae: Architecture of metabolic diversity. Proc. Natl. Acad. Sci. USA 95: 121 1 1 - 12116. Ziermann, R. and Betlach, M. (2000) A two-vector system for the production of recombinant polyketides in Streptomyces. J. Ind. Microbiol. Biotech. 24: 46-50. Ziermann, R. and Betlach, M.C. (1999) Recombinant polyketide synthesis in Streptomyces: Engineering of improved host strains. Biotechniques 26: 106-110. Example 2 Stabilizing meg PKS Expression Plasmid by Codon Engineering Materials and methods All bacterial strains were cultured and transformed as described in Example 1. Fermentation of Streptomyces and diketide feeding Primary Streptomyces transformants were picked and placed in 6 mL of TSB liquid medium with 50 μg/L of thiostrepton and grown at 30°C When the culture showed some growth (3-4days), it was transferred into a 250 mL flask containing 50 mL of R6 medium (pFI 7.0) with 25 ug/L of thiostrepton and lg/L of diketide ((2s,3R)2-methyl-3-hydroxyhexanoate N-propionyl cysteamine thioester) and placed in a 30°C incubator for 7 days. Changing codons and making plasm ids There are several identical sequences in the coding sequences for module 2 and module 6 of the megalomicin PKS gene cluster. Expression plasmids containing the full length megalomicin PKS appeared to be somewhat unstable + and subject to deletion in recA strains like ET 124567 and Streptomyces by intra- plasmid homologous recombination. To prevent significant homologous recombination and so stabilize expression plasmids, the codons of two regions of the module 6 coding sequence that are identical to regions in the module 2 coding sequence were changed without changing the sequence of protein encoded. The two regions changed in module 6 were from the 26739 lh base to 27,267 th base and from position 27,697 l base to 27,987 c base, which were identical to the region from position 6810 th base to 7338 th base and regions from position 7778 th base to 8068 th base, respectively. The start codon of the loading domain of the meg PKS was set to be the 1 st base. These sequences are shown below > 6810-7338 Sequence in Module 2 TTGCAGCGGTTGTCGGTGGCGGTGCGGGAGGGGCGTCGGGTGTTGGGTGTGGTGGTGGGT TCGGCGGTGAATCAGGATGGGGCGAGTAATGGGTTGGCGGCGCCGTCGGGGGTGGCGCAG CAGCGGGTGATTCGGCGGGCGTGGGGTCGTGCGGGTGTGTCGGGTGGGGATGTGGGTGTG GTGGAGGCGCATGGGACGGGGACGCGGTTGGGGGATCCGGTGGAGTTGGGGGCGTTGTTG GGGACGTATGGGGTGGGTCGGGGTGGGGTGGGTCCGGTGGTGGTGGGTTCGGTGAAGGCG AATGTGGGTCATGTGCAGGCGGCGGCGGGTGTGGTGGGTGTGATCAAGGTGGTGTTGGGG TTGGGTCGGGGGTTGGTGGGTCCGATGGTGTGTCGGGGTGGGTTGTCGGGGTTGGTGGAT TGGTCGTCGGG_GGG_TGGTGGTGGCGGA-GGGGTGCGGGGGTGGCCGGTGGGTGTGGAT GGGGTGCGTCGGGGTGGGGTGTCGGCGTTTGGGGTGTCGGGGACGAAT (SEQ ID NO: 23) > 26736-27267 Sequence in Module 6 CTGCAGCGGTTGTCGGTGGCGGTGCGGGAGGGGCGTCGGGTGTTGGGTGTGGTGGTGGGT TCGGCGGTGAATCAGGATGGGGCGAGTAATGGGTTGGCGGCGCCGTCGGGGGTGGCGCAG CAGCGGGTGATTCGGCGGGCGTGGGGTCGTGCGGGTGTGTCGGGTGGGGATGTGGGTGTG GTGGAGGCGCATGGGACGGGGACGCGGTTGGGGGATCCGGTGGAGTTGGGGGCGTTGTTG GGGACGTATGGGGTGGGTCGGGGTGGGGTGGGTCCGGTGGTGGTGGGTTCGGTGAAGGCG AATGTGGGTCATGTGCAGGCGGCGGCGGGTGTGGTGGGTGTGATCAAGGTGGTGTTGGGG TTGGGTCGGGGGTTGGTGGGTCCGATGGTGTGTCGGGGTGGGTTGTCGGGGTTGGTGGAT TGGTCGTCGGGTGGGTTGGTGGTGGCGGATGGGGTGCGGGGGTGGCCGGTGGGTGTGGAT GGGGTGCGTCGGGGTGGGGTGTCGGCGTTTGGGGTGTCGGGGACGAAT (SEQ ID NO: 24) > 26736-27267 Sequence with Codon Changes CTGCAGCGCCTCTCCGTCGCCGTCCGCGAGGGCCGCCGAGTCCTCGGCGTCGTCGTCGGC TCGGCCGTCAACCAAGACGGCGCGTCAAACGGCCTCGCCGCGCCCTCCGGCGTCGCCCAG CAGCGCGTCATACGCCGCGCGTGGGGACGCGCCGGAGTATCGGGCGGCGACGTCGGAGTC GTCGAGGCCCACGGCACCGGCACCCGCCTCGGGGATCCCGTCGAGCTGGGCGCCCTCCTG GGCACGTACGGCGTCGGCCGCGGCGGCGTCGGCCCGGTCGTCGTCGGCAGCGTCAAGGCC AACGTCGGCCACGTCCAGGCCGCGGCCGGCGTCGTCGGGGTCATCAAGGTCGTCCTCGGC CTCGGCCGCGGGCTGGTCGGCCCGATGGTCTGCCGCGGCGGCCTCAGCGGCCTCGTCGAC TGGTCGTCCGGCGGCCTGGTCGTCGCGGACGGGGTCCGCGGCTGGCCGGTCGGCGTCGAC GGCGTCCGCCGGGGCGGCGTCTCGGCGTTCGGCGTCAGCGGGACGAAT (SEQ ID NO: 25) > 6978-7337 Sequence in Module 2 GGTGGAGTGTGATGCGGTGGTGTCGTCGGTGGTGGGGTTTTCGGTGTTGGGGGTGTTGGA GGGTCGGTCGGGTGCGCCGTCGTTGGATCGGGTGGATGTGGTGCAGCCGGTGTTGTTCGT GGTGATGGTGTCGTTGGCGCGGTTGTGGCGGTGGTGTGGGGTTGTGCCTGCGGCGGTGGT GGGTCATTCGCAGGGGGAGATCGCGGCGGCGGTGGTGGCGGGGGTGTTGTCGGTGGGTGA TGGTGCGCGGGTGGTGGCGTTGCGGGCGCGGGCGTTGCGGGCGTTGGCCGG (SEQ ID NO: 26) > 27697-27987 Sequence in Module 6 GGTGGAGTGTGATGCGGTGGTGTCGTCGGTGGTGGGGTTTTCGGTGTTGGGGGTGTTGGA GGGTCGGTCGGGTGCGCCGTCGTTGGATCGGGTGGATGTGGTGCAGCCGGTGTTGTTCGT GGTGATGGTGTCGTTGGCGCGGTTGTGGCGGTGGTGTGGGGTTGTGCCTGCGGCGGTGGT GGGTCATTCGCAGGGGGAGATCGCGGCGGCGGTGGTGGCGGGGGTGTTGTCGGTGGGTGA TGGTGCGCGGGTGGTGGCGTTGCGGGCGCGGGCGTTGCGGGCGTTGGCCGG (SEQ ID NO: 27) > 27697-27987 Sequence with Codon Changes CGTGGAGTGCGATGCGGTCGTGTCGAGCGTCGTCGGCTTCAGCGTGCTGGGCGTCCTGGA GGGCCGCAGCGGCGCCCCGAGCCTGGACCGCGTCGACGTGGTCCAGCCGGTCCTGTTCGT GGTCATGGTCAGCCTGGCCCGCCTGTGGCGCTGGTGCGGCGTGGTCCCGGCCGCCGTGGT CGGCCACAGCCAGGGCGAGATCGCCGCCGCGGTCGTGGCCGGCGTCCTGAGCGTCGGCGA CGGCGCCCGCGTCGTGGCCCTGCGCGCCCGCGCCCTGCGCGCCCTGGCCGG (SEQ ID NO: 28) Three pieces of DNA from the two regions above were synthesized and verified by Retrogen, and the synthesized DNAs were cloned into pCR-Blunt II -TOPO, as shown in the Table 3 below. Table 3. Plasmids containing synthesized DNA Assembly of the expression plasmid First, ligation of the Pstl-BamHI fragment of pKOS97-1613, the BamPlI- Bsml fragment of pKOS97-1622 and Bsml-Pstl linearized pKOS97-90 produced pKOS97-151. Then, the insertion of the SfaNl-Fsel fragment of pKOS97-l628 into pKOS97-151 gave rise to pKSO97-152. Then, the Pstl-Blpl fragment of pKOS97-125 was used to replace the Pstl-Blpl fragment of pKOS97-90a and produced pKOS97- 160. The final expression plasmid (in pRM5) pKOS97-162 was the result of Bglll-Nhel fragment of pKOS97-160 inserted into Bglll-Nhel sites of pKOS 108- 04. Another expression plasmid pKOS97-152a was made by a four-fragment ligation. The four fragments were a Blpl-Xbal fragment (containing a cos site) of pKOS97-92a, a Bglll-Pstl fragment of pKOS97-81 , a Pstl-Blpl fragment of pKOS97-152, and a Bglll-Xbal fragment of pKOS108-04 (as the vector). Tests of the constructed plasmids showed that the plasmids containing the modified coding sequences were more stable than plasmids containing unmodified coding sequence. Example 3 Construction of Ole-Meg Hybrid PKS Construction of pRMl -based pKOS098-48 for the expression ofOlePKS modules 1-4. The 240-bp fragment containing the 3 '-end portion of oleAII gene (at nt 1 1210-11452; the first base of the start codon of oleAII is nt 1) was PCR amplified with primers N98-38-1 (5 ' GAAC AACTCCTGTCTGCGGCCGCG-3 ') (SEQ ID NO: 29) and N98-38-3 (5'- CGGAATTCTCTAGAGTCACGTCTCCAACCGCTTGTCGAGG-3') (SEQ ID NO: 30). The fragment contains a naturally occurring Notl site at its 5'-end and the engineered Xbal (bold) and EcoRI sites (underline) at its 3 '-end following the oleAII stop codon. pKOS38-189 was digested with EcoRI and Notl to give five fragments of 8 kb, 5 kb, 4 kb, 2.5 kb and 2 kb. The 8-kb EcoRl-Notl fragment containing oleA/I gene nt 2961 to nt 1 1210 and the 240-bp Notl, EcoRI treated PCR fragment were ligated into litmus 28 at the EcoRI site via a three-fragment ligation to give pKOS98-46. The 8.2-kb EoRI fragment from pKOS98-46 was cloned into pKOS38-174, a pRMl derived plasmid containing oleAI and nt 1 to nt 2960 of oleAII to give pKOS98-48. Construction ofpSET152-basedpKOS98-60for the expression ofmegPKS modules 5-6. The 360-bp fragment containing nt 1 to nt 366 of megAIll was PCR amplified with primers N98-40-3 (5'- TCTAGACTTAATTAAGGAGGACAC- TA TGAGCG A-GAGC AGC- GGCATGACCG-3') (SEQ ID NO: 31) and N98-40-2 (5'- AACGCCTCCCAG- GAGATCTCCAGCA-3') (SEQ ID NO: 32). A Pad site and a Ndel site as well as the ribosome binding site were introduced at the 5 '-end of the megAI start codon. The 360-bp PacI-BgUI fragment was inserted into pKOS 108-06 replacing the 22-kb PacI-BgUI fragment to yield pKOS98-55. The 10-kb Pacl-Xbal fragment containing megAIll gene and the annealed oligos N98-23-1 (5'- AATTCATAGCCTAGGT-3') (SEQ ID NO: 33) and N98-23-2 (5'- CTAGACCTAGGCTATG-3') (SEQ ID NO: 34) were ligated to Pad and EcoRI treated pSETl 52 derivative pKOS98-l 4 via a three- fragment ligation to give pKOS98-60. Example 4 Conversion of Ervthronolides to Erythromycins A sample of a polyketide (-50 to 100 mg) is dissolved in 0.6 mL of ethanol and diluted to 3 L with sterile water. This solution is used to overlay a three day old culture of Saccharopolyspora erythraea WHM34 (an ery A mutant) grown on a 100 mm R2YE agar plate at 30°C. After drying, the plate is incubated at 30°C for four days. The agar is chopped and then extracted three times with 100 mL portions of 1% triethylamine in ethyl acetate. The extracts are combined and evaporated. The crude product is purified by preparative HPLC (C-18 reversed phase, water-acetonitrile gradient containing 1% acetic acid). Fractions are analyzed by mass spectrometry, and those containing pure compound are pooled, neutralized with triethylamine, and evaporated to a syrup. The syrup is dissolved in water and extracted three times with equal volumes of ethyl acetate. The organic extracts are combined, washed once with saturated aqueous NaHCO 3 , dried overNa 2 SO , filtered, and evaporated to yield -0.15 mg of product. The product is a glycosylated and hydroxylated compound corresponding to erythromycin A, B, C, and D but differing therefrom as the compound provided differed from 6-dEB. Example 5 Measurement of Antibacterial Activity Antibacterial activity is determined using either disk diffusion assays with Bacillus cereus as the test organism or by measurement of minimum inhibitory concentrations (MIC) in liquid culture against sensitive and resistant strains of Staphylococcus pneumoniae. Example 6 Evaluation of Antiparasitic Activity Compounds can initially screened in vitro using cultures of P. falciparum FCR-3 and Kl strains, then in vivo using mice infected with P. berghei. Mammalian cell toxicity can be determined in FM3A or KB cells. Compounds can also be screened for activity against P. berhei. Compounds are also tested in animal studies and clinical trials to test the antiparasitic activity broadly (antimalarial, trypanosomiasis and Leishmaniasis). The invention having now been described by way of written description and example, those of skill in the art will recognize that the invention can be practiced in a variety of embodiments and that the foregoing description and examples are for purposes of illustration and not limitation of the following claims.

Claims: Claims 1. An isolated nucleic acid comprising a nucleotide sequence encoding a domain of megalomicin polyketide synthase (PKS) or a megalomicin modification enzyme. 2. The isolated nucleic acid of claim 1 , which encodes a PKS open reading frame (ORF) selected from the group consisting of megAI, megAIl and megAIll. 3. The isolated nucleic acid of claim 1, wherein the PKS domain is selected from the group consisting of a TE domain, a KS domain, an AT domain, an ACP domain, a KR domain, a DH domain, and an ER domain. 4. The isolated nucleic acid of claim 1, wherein the nucleic acid comprises the coding sequence for a loading module, a thioesterase domain, and all six extender modules of megalomicin PKS. 5. The isolated nucleic acid of claim 1, which encodes a megalomicin modification enzyme that is involved in the conversion of 6-dEB into a megalomicin. 6. The isolated nucleic acid of claim 5, which encodes a megalomicin modification enzyme that is involved in the biosynthesis of mycarose, megosamine or desosamine. 7. The isolated nucleic acid of claim 1 , wherein the nucleic acid codons of homologous regions within the PKS or the megalomicin modification enzyme coding sequence have been changed to reduce or abolish the homology without changing the amino acid sequences encoded by said changed nucleic acid codons. 8. The isolated nucleic acid of claim 1, which isolated nucleic acid fragment hybridizes to a nucleic acid having a nucleotide sequence set forth in the SEQ. ID NOT . 9. A polypeptide, which is encoded by the isolated nucleic acid fragment of claim 1. 10. A recombinant DNA expression vector, comprising the isolated nucleic acid of claim 1 operably linked to a promoter. 1 1. A recombinant host cell, comprising the recombinant DNA expression vector of claim 10. 12. The recombinant host cell of claim 11 , which is a Streptomyces or Saccharopolyspora host cell. 13. A recombinant host cell of claim 11, which comprises: a) at least two separate autonomously replicating recombinant DNA expression vectors, each of said vectors comprises a recombinant DNA compound encoding a megalomicin PKS domain or a megalomicin modification enzyme operably linked to a promoter; or b) at least one autonomously replicating recombinant DNA expression vector and at least one modified chromosome, each of said vector(s) and each of said modified chromosome comprises a recombinant DNA compound encoding a megalomicin PKS domain or a megalomicin modification enzyme operably linked to a promoter. 14. A hybrid PKS that comprises a polypeptide of claim 9 and is composed of at least a portion of a megalomicin PKS and at least a portion of a second PKS for a polyketide other than megalomicin. 15. The hybrid PKS of claim 14, wherein the second PKS is selected from the group consisting of a narbonolide PKS, an oleandolide PKS, and a DEBS PKS. 16. The hybrid PKS of claim 15 that is composed of the megAI and megAIl gene products and the oleAIII gene product. 17. The hybrid PKS of claim 16, wherein the KS domain of module 1 of the megAI gene product has been inactivated by mutation. 18. A method of producing a polyketide, which method comprises growing the recombinant host cell of claim 11 under conditions whereby the megalomicin PKS domain encoded by the recombinant expression vector is produced and the polyketide is synthesized by the cell, and recovering the synthesized polyketide. 19. A recombinant host cell that comprises a recombinant expression vector that encodes a megalomicin modification enzyme. 20. The recombinant host cell of claim 19 that produces megosamine and can attach megosamine to a polyketide, wherein said host cell, in its naturally occurring non-recombinant state cannot produce megosamine.

Inventors: MCDANIEL ROBERT, VOLCHEGURKSY YANINA, MCDANIEL, ROBERT, VOLCHEGURKSY, YANINA
Applicants: KOSAN BIOSCIENCES INC, KOSAN BIOSCIENCES, INC.
Application year: 2000
Main classification: C12N15
IPC codes: C12N1, C12N5, C12N9, C12N15, C12P19