# Patent

## Title
Midecamycin biosynthetic genes

## Abstract

The present invention provides an isolated polypeptide comprising a nucleotide sequence encoding a protein which is involved in midecamycin biosynthesis, wherein the protein contains an amino acid sequence selected from SEQ ID NOs: 2 to 10, 13, 14, 16, 19, 20, 22 to 26, and 28 to 38 or a modified amino acid sequence of the amino acid sequence having one or more amino acid modifications without affecting activity of the protein.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to midecamycin biosynthesis genes which are involved in the production of midecamycins, and more specifically to genes encoding functional modules of polyketide synthases.
2. Background Technology
Since macrolide antibiotics which are effective to gram-positive bacteria, mycoplasms, chlamydias and the like can be orally administered and have low toxicity, they are classified as clinically important antibiotics. In particular, commercially-available 16-membered ring macrolide antibiotics are widely used in the world, mainly in Asian countries, because of their advantages, for example, that they are less likely to induce resistant strains and less interactive with other drugs than 14-membered ring macrolides, and have little effect on the intestinal tract.
Midecamycins (FIG. 1) belong to 16-membered ring macrolide antibiotics and several analogues have been reported. They are clinically used extensively along with miokamycin, an acylated derivative of a midecamycin (Omoto, S. et al., J. Antibiot., 29, 536 (1976); Yoshida, T. et al., Jpn. J. Antibiot., 35, 1462 (1982)).
Midecamycins are produced by a species of actinomycetes, Streptomyces mycarofaciens (ATCC 21454), and industrial scale production by fermentation using this strain has been established. Conventionally, actinomycetes have an important role in the field of fermentation industry as microorganisms for the production of secondary metabolic products, such as antibiotics and physiologically active substances, and their productivity has been improved by various microbial breeding techniques. The microbial breeding has also been carried out for midecamycin production by Streptomyces mycarofaciens by inducing mutation with various mutagens.
Recently, recombinant DNA technology has been introduced to improve productivity of secondary metabolites and to create novel active substances and a number of genes in secondary metabolic systems have already been isolated. Examples of isolated genes involved in the production of macrolide antibiotics include tylosin biosynthesis genes (Merson-Davies, L. A. and Cundliffe, E., Mol. Microbiol., 13, 349 (1994); Gandecha, A. R. et al., Gene, 184, 197 (1997); Wilson, V. T. and Cundliffe, E., Gene, 214, 95 (1998); Fouces, R. et al., Microbiology, 145, 855 (1999); Bate, N. et al., Microbiology, 146, 139 (2000); Review: Cundliffe, E. et al., Antonie Van Leeuwenhoek, 79, 229 (2001); U.S. Pat. Nos. 5,876,991, 5,672,497, 5,149,638, European Patent No. 791655, European Patent No. 238323), nidamycin biosynthesis genes (Kakavas, S. J. et al., J. Bacteriol., 179, 7515 (1997); WO98/51695), and erythromycin biosynthesis genes (Dhillon, N. et al., Mol. Microbiol., 3, 1405 (1989); Cortes, J. et al., Nature, 348, 176 (1990); Donadio, S. et al., Science, 252, 675 (1991); Haydock, S. F. et al., Mol. Gen. Genet., 230, 120 (1991); Stassi, D. et al., J. Bacteriol., 175, 182 (1993); Linton, K. J. et al., Gene, 153, 33 (1995); Gaisser, S. et al., Mol. Gen. Genet., 256, 239 (1997); Summers, R. G. et al., Microbiology, 143, 3251 (1997); Gaisser, S. et al., Mol. Gen. Genet., 258, 78 (1998); Salah-Bey, K. et al., Mol. Gen. Genet., 257, 542 (1998); WO93/13663, U.S. Pat. Nos. 6,004,787, 5,824,513, WO97/23630, U.S. Pat. No. 5,998,194).
In microorganisms which produce macrolide antibiotics, most of the macrolide biosynthesis genes are often clustered together in a region of 70 to 80 kb in the genome (Donadio, S. et al., Science, 252, 675 (1991); MacNeil, D. J. et al., Gene, 115, 119 (1992); Schwecke, T. et al., Proc. Natl. Acad. Sci., 92, 7839 (1995)). In the center of such clusters, there exists a highly homologous gene called Type I polyketide synthase (PKS) which encodes a huge multi-functional protein.
The PKS is generally composed of 3 to 5 genes and its protein forms a complex comprising an initiator module and several extender modules. Each of these components adds a specific acyl-CoA precursor to a polyketide chain in the process of synthesis to specifically modify β-keto groups. Accordingly, the structure of polyketide is determined by the composition and the order of these modules in the PKS. The modules contain several domains and each of them has its specific function.
The initiator module is composed of an acyl-carrier protein (ACP) domain to which an acyl group of precursor binds and an acyltransferase (AT) domain which catalyzes addition of the acyl group to the ACP domain. Difference in specificity of this AT domain determines the kind of acyl-CoA to be added thereto. All of the extender modules contain a β-ketosynthase (KS) domain, which adds a previously existing polyketide chain to a new acyl-ACP by decarboxylation condensation, the AT domain and the ACP domain.
Further, in addition to these domains, the extender modules contain several domains which modify specific β-keto groups and the composition of the domains contained determines the modification of β-keto groups. Such domains include a β-ketoreductase (KR) domain which reduces a β-keto group to a hydroxyl group, a dehydratase (DH) domain which removes a dehydroxyl group and generates a double bond, and an enoylreductase (ER) domain which reduces a double bond and generates a saturated carbon bond.
The last extender module ends with a thioesterase (TE) domain which catalyzes the cyclization and release of polyketide from the PKS.
A polyketide skeleton produced by PKS undergoes further modifications, such as methylation, acylation, oxidation, reduction, and addition of specific sugars, to ultimately synthesize macrolide antibiotics. Most of the genes necessary for these modifications exist in the vicinity of the PKS gene.
As for genes involved in midecamycin biosynthesis, a midecamycin self-resistance gene (mdmA; Hara, O. and Hutchinson, C. R., J. Antibiot., 43, 977 (1990)), a 3-O-acyltransferase gene (mdmB), an O-methyltransferase gene (mdmC; Hara, O. and Hutchinson, C. R., J. Bacteriol., 174, 5141 (1992)), and a 4′′-O-propionyltransferase gene (mpt; Xulun, Z. and Yiguang, W., Acta Microbiol. Sci., 36, 417 (1996)) have been reported. However, no other gene involved in midecamycin biosynthesis has been reported.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a midecamycin biosynthesis gene, a recombinant vector having said gene and a host having said recombinant vector.
The present invention provides an isolated polynucleotide comprising a nucleotide sequence encoding a protein which is involved in midecamycin biosynthesis, wherein said protein comprises an amino acid sequence selected from the group consisting of the following sequences (hereinafter referred to as “midecamycin biosynthesis gene”):
(a) an amino acid sequence selected from SEQ ID NOs: 2 to 10, 13, 14, 16, 19, 20, 22 to 26, and 28 to 38,
(b) an amino acid sequence of a protein involved in biosynthesis of midecamycin, which is encoded by a clone contained in the microorganism deposited under an accession number of FERM BP-8168,
(c) an amino acid sequence of a protein involved in biosynthesis of midecamycin, which is encoded by a clone contained in the microorganism deposited under an accession number of FERM BP-8169,
(d) an amino acid sequence of a protein involved in biosynthesis of midecamycin, which is encoded by a clone contained in the microorganism deposited under an accession number of FERM BP-8170, and
(e) a modified amino acid sequence of (a), (b), (c), or (d) having one or more amino acid modifications without affecting activity of the protein.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the structures of midecamycins Al, A2, A3, B, DH, E, and CH3.
FIG. 2 shows the positions of cosmid clones pCOMW1, pCOMW2, and pCOMW4 on the ORFs.
FIG. 3 shows the positions of the ORFs determined in the present invention.
FIG. 4 shows the biosynthesis pathways for the polyketide skeleton precursors.
FIG. 5 shows the biosynthesis pathways for the polyketide skeleton. M: malonyl-CoA, MM: methylmalonyl-CoA, EM: ethylmalonyl-CoA, MOM: methoxymalonyl-CoA.
FIG. 6 shows the biosynthesis pathway for the deoxy sugars.
FIG. 7 shows the modification system for the polyketide skeleton.
FIG. 8 shows the positions of each domain and module in the PKS. KS: β-ketosynthase, AT: acyltransferase, DH: dehydratase, ER: enoylreductase, KR: β-ketoreductase, ACP: acyl-carrier protein, TE: thioesterase, null: no function.



DETAILED DESCRIPTION OF THE INVENTION
Definitions
In the present invention, the term “modification” refers to a substitution, a deletion, an addition and an insertion.
The term “one or more amino acid modifications” herein refers to modifications which do not substantially change protein activity. The number of amino acid residues to be modified is preferably 1 to 40, more preferably one to several, further more preferably 1 to 8, and most preferably 1 to 4.
An example of the “modifications without affecting activity” in the present invention includes a conservative substitution. The term “conservative substitution” means the substitution of one or more amino acid residues with other chemically homologous amino acid residues so as not to substantially change protein activity. For example, a certain hydrophobic residue can be substituted with another hydrophobic residue and a certain polar residue can be substituted with another polar residue having the same charge. Functionally homologous amino acids capable of carrying out these substitutions for each amino acid are known to those skilled in the art. More specifically, examples of the non-polar (hydrophobic) amino acids include alanine, valine, isoleucine, leucine, proline, tryptophan, phenylalanine, and methionine. Examples of the polar (neutral) amino acids include glycine, serine, threonine, tyrosine, glutamine, asparagine, and cysteine. Examples of the positively charged (basic) amino acids include arginine, histidine, and lysine. Examples of the negatively charged (acidic) amino acids include aspartic acid and glutamic acid.
Deposition of Microorganisms
Escherichia coli transformed with pCOMW1 was deposited with the International Patent organism Depositary, National Institute of Advanced Industrial Science and Technology (AIST Tsukuba Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8566 Japan), dated Jul. 16, 2002. The accession number is FERM BP-8168.
Escherichia coli transformed with pCOMW2 was deposited with the International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology (AIST Tsukuba Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8566 Japan), dated Jul. 16, 2002. The accession number is FERM BP-8169.
Escherichia coli transformed with pCOMW4 was deposited with the International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology (AIST Tsukuba Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8566 Japan), dated Jul. 16, 2002. The accession number is FERM BP-8170.
Midecamycin Biosynthesis Gene
Functions of proteins comprising amino acid sequences selected from SEQ ID NOs: 2 to 10, 13, 14, 16, 19, 20, 22 to 26, and 28 to 38 encoded by a midecamycin biosynthesis gene according to the present invention are as described in Table 2 hereinafter.
Nucleotide sequences encoding these proteins can be, for example, nucleotide sequences selected from bases















29244–42779,
42823–48657,
48712–59802,
59850–64556,


64687–70365,
70365–71078,
71113–72360,
72400–73665,


73694–75043,
78039–79313,
79391–81052,
82760–83362,


27937–28983,
26180–27391,
24460–25650,
23555–24463,


22534–23571,
21733–22527,
20307–21743,
17522–18895,


15643–17466,
14074–15096,
13016–14044,
11729–12961,







10521–11603, 9328–10458, 9012–9335, 8149–9015, 6653–7945,


and 6048–6629 of SEQ ID NO: 1.









A midecamycin biosynthesis gene according to the present invention can be a polynucleotide comprising a nucleotide sequence which can hybridize with a nucleotide sequence which encodes an amino acid sequence selected from SEQ ID NOs: 2 to 10, 13, 14, 16, 19, 20, 22 to 26, and 28 to 38, under stringent conditions. The term “hybridize” in the present invention means to hybridize with a target nucleotide sequence but not with a nucleotide other than the target nucleotide under stringent conditions. The term “stringent conditions” means that the membrane washing after hybridization is carried out in a low salt solution at a high temperature, for example, at a concentration of 0.2×SSC (1×SSC: 15 mM trisodium citrate, 150 mM sodium chloride) in a 0.1% SDS solution at 60° C. for 15 minutes.
A polyketide synthase involved in midecamycin biosynthesis comprises a complex of several modules and each module has several functional domains. Accordingly, the present invention provides an isolated polynucleotide comprising a nucleotide sequence encoding a functional domain of polyketide synthase (PKS) which is involved in midecamycin biosynthesis, wherein said domain comprises an amino acid sequence selected from the group consisting of the following sequences (1) to (9):
(1) an amino acid sequence selected from amino acid residues 17–422 (KS0null), 524–878 (AT0), 919–1004 (ACP0), 1031–1456 (KS1), 1562–1916 (AT1), 2161–2449 (KR1), 2475–2560 (ACP1), 2583–3008 (KS2), 3129–3483 (AT2), 3499–3699 (DH2), 4022–4315 (KR2), and 4333–4418 (ACP2) of SEQ ID NO: 2,
(2) an amino acid sequence selected from amino acid residues 35–460 (KS3), 577–929 (AT3), 943–1169 (DH3), 1457–1744 (KR3), and 1759–1844 (ACP3) of SEQ ID NO: 3,
(3) an amino acid sequence selected from amino acid residues 42–467 (KS4), 568–916 (AT4), 1137–1408 (KR4null), 1417–1502 (ACP4), 1522–1948 (KS5), 2064–2414 (AT5), 2426–2618 (DH5), 2939–3229 (ER5), 3219–3504 (KR5), and 3520–3605 (ACP5) of SEQ ID NO: 4,
(4) an amino acid sequence selected from amino acid residues 34–458 (KS6), 563–914 (AT6), 1134–1418 (KR6), and 1427–1509 (ACP6) of SEQ ID NO: 5,
(5) an amino acid sequence selected from amino acid residues 35–460 (KS7), 576–929 (AT7), 1217–1500 (KR7), 1504–1591 (ACP7), and 1588–1892 (TE7) of SEQ ID NO: 6,
(6) an amino acid sequence of a functional domain of PKS involved in midecamycin biosynthesis, which is encoded by a clone contained in the microorganism deposited under an accession number of FERM BP-8168,
(7) an amino acid sequence of a functional domain of PKS involved in midecamycin biosynthesis, which is encoded by a clone contained in the microorganism deposited under an accession number of FERM BP-8169,
(8) an amino acid sequence of a functional domain of PKS involved in midecamycin biosynthesis, which is encoded by a clone contained in the microorganism deposited under an accession number of FERM BP-8170, and
(9) an amino acid sequence of any one of (1) to (8) having one or more amino acid modifications without affecting activity of said domain.
The present invention also provides an isolated polynucleotide comprising a nucleotide sequence encoding a functional domain of polyketide synthase (PKS) which is involved in midecamycin biosynthesis, wherein said nucleotide sequence is selected from the group consisting of the following sequences (10) to (14):
(10) a nucleotide sequence which can hybridize with a nucleotide sequence encoding an amino acid sequence selected from amino acid residues 17–422 (KS0null), 524–878 (AT0), 919–1004 (ACP0), 1031–1456 (KS1), 1562–1916 (AT1), 2161–2449 (KR1), 2475–2560 (ACP1), 2583–3008 (KS2), 3129–3483 (AT2), 3499–3699 (DH2), 4022–4315 (KR2), and 4333–4418 (ACP2) of SEQ ID NO: 2, under stringent conditions,
(11) a nucleotide sequence which can hybridize with a nucleotide sequence encoding an amino acid sequence selected from amino acid residues 35–460 (KS3), 577–929 (AT3), 943–1169 (DH3), 1457–1744 (KR3), and 1759–1844 (ACP3) of SEQ ID NO: 3, under stringent conditions,
(12) a nucleotide sequence which can hybridize with a nucleotide encoding an amino acid sequence selected from amino acid residues 42–467 (KS4), 568–916 (AT4), 1137–1408 (KR4null), 1417–1502 (ACP4), 1522–1948 (KS5), 2064–2414 (AT5), 2426–2618 (DH5), 2939–3229 (ER5), 3219–3504 (KR5), and 3520–3605 (ACP5) of SEQ ID NO: 4, under stringent conditions,
(13) a nucleotide sequence which can hybridize with a nucleotide sequence encoding an amino acid sequence selected from amino acid residues 34–458 (KS6), 563–914 (AT6), 1134–1418 (KR6), and 1427–1509 (ACP6) of SEQ ID NO: 5, under stringent conditions, and
(14) a nucleotide sequence which can hybridize with a nucleotide sequence encoding an amino acid sequence selected from amino acid residues 35–460 (KS7), 576–929 (AT7), 1217–1500 (KR7), 1504–1591 (ACP7), and 1588–1892 (TE7) of SEQ ID NO: 6, under stringent conditions.
A polynucleotide encoding a domain comprising amino acid sequence (1) can be a nucleotide sequence selected from bases 29292–30509, 30813–31877, 31998–32255, 32334–33611, 33927–34991, 35724–36590, 36666–36923, 36990–38267, 38628–39692, 39738–40340, 41307–42188, and 42240–42497 of SEQ ID NO: 1.
A polynucleotide encoding a domain comprising amino acid sequence (2) can be a nucleotide sequence selected from bases 42925–44202, 44551–45609, 45649–46329, 47191–48054, and 48097–48354 of SEQ ID NO: 1.
A polynucleotide encoding a domain comprising amino acid sequence (3) can be a nucleotide sequence selected from bases 48835–50112, 50413–51459, 52120–52935, 52960–53217, 53275–54555, 54901–55953, 55987–56565, 57526–58398, 58366–59223, and 59269–59526 of SEQ ID NO: 1.
A polynucleotide encoding a domain comprising amino acid sequence (4) can be a nucleotide sequence selected from bases 59949–61223, 61536–62591, 63249–64103, and 64128–64376 of SEQ ID NO: 1.
A polynucleotide encoding a domain comprising amino acid sequence (5) can be a nucleotide sequence selected from bases 64789–66066, 66412–67473, 68335–69186, 69196–69459, and 69448–70362 of SEQ ID NO: 1.
Isolation of Midecamycin Biosynthesis Gene
A midecamycin biosynthesis gene according to the present invention can be isolated, for example, from Streptomyces mycarofaciens (ATCC 21454) or its mutant strains by the following method. Further, a pertinent gene can be artificially synthesized since its sequence is known as disclosed in the present invention.
A genomic DNA is extracted from cells of Streptomyces mycarofaciens by a conventional method described in Kieser, T. et al., Practical Streptomyces Genetics, The John Innes Foundation, Norwick, UK (2000). This genomic DNA is digested with an appropriate restriction enzyme and then ligated with an appropriate vector to construct a genomic library comprising a genomic DNA of Streptomyces mycarofaciens. Various vectors such as plasmid vectors, phage vectors, cosmid vectors, and BAC vectors can be used as a vector.
Next, appropriate probes are made based on the sequence of the midecamycin biosynthesis gene disclosed in this specification, hybridization is carried out and then a DNA fragment which contains the target midecamycin biosynthesis gene can be obtained from the resulting genomic library. Alternatively, appropriate primers for amplification of the gene of interest are synthesized based on the sequence of the midecamycin biosynthesis gene disclosed in this specification, PCR is carried out using the genomic DNA of Streptomyces mycarofaciens as a template, and then the target gene can be isolated by ligating the amplified DNA fragment with an appropriate vector. The DNA fragment containing the midecamycin biosynthesis gene according to the present invention is contained in pCOMW1, pCOMW2, and pCOMW4 in a ligated form with cosmid vectors (FIG. 2), which can be used as a template for the PCR. Further, the desired DNA fragment can be excised from these deposited cosmid vectors using an appropriate restriction enzyme.
In this way, the polyketide synthesis enzyme gene of Streptomyces mycarofaciens and its neighboring regions can be isolated.
It is possible to confirm whether the isolated DNA fragment contains the midecamycin biosynthesis gene by constructing a strain having a specific gene disruption by incorporating a vector containing an internal fragment of the target gene or a vector having a selectable marker gene insert, which divides the internal part of the target gene, to induce homologous recombination and then by evaluating no production of midecamycin from this gene disruption strain when cultured. Midecamycin can be detected by extracting from a culture fluid with an appropriate organic solvent and analyzing the extract using HPLC. Midecamycin can also be detected by treating the culture fluid with midecamycin-sensitive bacteria and examining the growth of the bacteria.
Transformants
In order to improve productivity by recombinant DNA technology, enhancement of expression of a gene which encodes a rate-limiting biosynthesis reaction, enhancement of expression of a gene which controls expression of a biosynthesis gene, gene disruption, blocking of unnecessary secondary metabolic systems, and the like have been carried out (Kennedy, J. and Turner, G., Mol. Gen. Genet., 253, 189 (1996); Review: Baltz, R. H., Biotechnology of Antibiotics Second Edition, Revised and Expanded, Marcel Dekker, Inc., NewYork, pp.49 (1997); Review: Hutchinson, C. R. and Colombo, A. L., J. Ind. Microbiol. Biotechnol., 23, 647 (1999); Review: Brakhage, A. A., Microbiol. Mol. Biol. Rev., 62, 547 (1998)). Accordingly, if a biosynthesis gene is specified, productivity can be improved by recombinant DNA technology by ligating the gene with an appropriate vector and introducing the vector into a microorganism for producing a secondary metabolite.
On the other hand, in order to create novel active substances by recombinant DNA technology, modifications of domains for polyketide synthesizing enzymes (Review: Ikeda and Omura, Protein, Nucleic Acid and Enzyme, 43, 1265 (1998); Review: Carreras, C. W. and Santi, D. V., Curr. Opin. Biotech., 9, 403 (1998); Review: Hutchinson, C. R., Curr. Opin. Microbiol., 1, 319 (1998); Review: Katz, L. and McDaniel, R., Med. Res. Rev., 19, 543 (1999); WO93/13663, WO95/08548, WO96/40968, WO98/01546, WO98/49315, WO98/51695, WO00/47724, U.S. Pat. Nos. 5,672,491, 5,712,146, 639,159), disruption of genes of biosynthesis systems, introduction of modified enzyme genes from other organisms (Review: Hutchinson, C. R., Biotechnology, 12, 375 (1994)), and the like have been carried out. Accordingly, if a biosynthesis gene is specified, a novel active substance can be produced by recombinant DNA technology by ligating the gene with an appropriate vector and introducing the vector into a microorganism for producing a secondary metabolite.
Thus, according to the present invention, productivity of midecamycin can be improved by ligating a midecamycin biosynthesis gene according to the present invention and a gene encoding a functional module with an appropriate vector and introducing the vector into a host such as Streptomyces mycarofaciens to enhance or control its expression, or by disrupting functions of domains in the gene by gene disruption using homologous recombination. Also, according to the present invention, a macrolide compound other than midecamycin can be produced by ligating a midecamycin biosynthesis gene according to the present invention and a gene encoding a functional module with an appropriate vector and introducing the vector into a host such as Streptomyces mycarofaciens to enhance or control its expression, or by disrupting functions of domains or substituting domains in the gene.
A recombinant vector for gene transfer can be constructed by modifying a polynucleotide provided by the present invention into an appropriate form depending on the purpose using a conventional method in the recombinant DNA technology, for example, described in Sambrook, J. et al., Molecular Cloning: a laboratory manual, Cold Spring Harbor Laboratory, New York (1989) and ligating it with a vector.
Vectors to be used in the present invention can be appropriately selected from viruses, plasmids, cosmid vectors, and the like, taking the kind of host cells to be used into consideration. For example, lambda bacteriophages and pBR322 and pUC vectors can be used for Escherichia coli; pUB110, pPL603, and pC194 vectors can be used for Bacillus subtilis; pYC and pYE vectors can be used for yeasts; and pIJ101, pSET152, pSG5, SCP2 *, pSAM2, pKC1139, and φC31 vectors can be used for actinomycetes (Kieser, T. et al., Practical Streptomyces Genetics, The John Innes Foundation, Norwick, UK (2000)).
Among the plasmid vectors to be used, at least one vector preferably contains a selectable marker to select transformants. A drug resistance gene or a gene complementing a nutritional requirement can be used as a selectable maker. Preferable examples of the marker genes to be used for each host include an ampicillin resistance gene, a kanamycin resistance gene, and a tetracycline resistance gene for bacteria; a tryptophan biosynthesis gene (TRP1), an uracyl biosynthesis gene (URA3), and a leucine biosynthesis gene (LEU2) for yeasts; a hygromycin resistance gene, a bialaphos resistance gene, a bleomycin resistance gene, and an aureobacidin resistance gene for fungi; and a kanamycin resistance gene and a bialaphos resistance gene for plants.
Further, in an expression vector, regulatory sequences necessary for expression of each gene, for example, transcription regulatory signals and translation regulatory signals, such as a promoter, a transcription initiation signal, a ribosome binding site, a translation stop signal, and a transcription stop signal, can operably be linked to the biosynthesis gene. The regulatory sequences can be selected and ligated according to an ordinary method.
For example, promoters such as a lactose operon and a tryptophan operon can be used for Escherichia coli; promoters such as an alcohol dehydrogenase gene, an acid phosphatase gene, a galactose utilization gene, and a glyceraldehyde triphosphate dehydrogenase gene can be used for yeasts; promoters such as an α-amylase gene, a glucoamylase gene, a cellobiohydrolase gene, a glyceraldehyde triphosphate dehydrogenase gene, and an Abp1 gene can be used for fungi; and the CaMV 35S RNA promoter and CaMV 19S RNA promoter, and a noparin synthase gene promoter can be used for plants.
A host for gene transfer can be appropriately selected from actinomycetes, Escherichia coli, Bacillus subtilis, yeasts, filamentous fungi and other microorganisms depending on the kind of vectors to be used. When the vector is for actinomycetes, examples of particularly preferable hosts include Streptomyces mycarofaciens, Streptomyces coelicolor, Streptomyces hygroscopicus, Streptomyces fradiae, Streptomyces lividans, Streptomyces kitasatoensis, Streptomyces ambofaciens, and Streptomyces thermotolerans. 
A method of introducing a vector into a host microorganism is selected to be most efficient depending on a vector and host to be used. When a vector for actinomycetes is used, transfer by conjugation with Escherichia coli, infection with an actinomycetes phage, introduction into the protoplast of the host, or the like can be carried out (Kieser, T. et al., Practical Streptomyces Genetics, The John Innes Foundation, Norwick, UK (2000)). For the selection of recombinants obtained by transformation, genetic indices carried by vectors to be used, such as antibiotic resistance, pock formation, and melanin biosynthesis, can be utilized.
In the present invention, when multiple biosynthesis genes are introduced into a host, each gene can be contained in the same or different DNA molecules. Further, when the host is a bacterium, it is possible to design each gene to be expressed as a polycistronic mRNA and thus make into one DNA molecule.
Gene disruption using homologous recombination can be carried out according to a conventional method. Construction of vectors for the gene disruption and introduction of the vectors into the host are known to the skilled in the art.
Transformants thus obtained are cultured and newly acquired properties can be examined according to a conventional method. As a medium, conventional components can be used. For example, as a carbon source, glucose, sucrose, starch syrup, dextrin, starch, glycerol, molasses, animal and vegetable oils, and the like can be used. As a nitrogen source, soybean powder, wheat germ, cornsteep liquor, cottonseed lees, meat extract, polypeptone, malt extract, yeast extract, ammonium sulfate, sodium nitrate, urea, and the like can be used. If necessary, inorganic salts which can produce sodium, potassium, calcium, magnesium, cobalt, chlorine, phosphoric acid (e.g., dipotassium hydrogenphosphate), sulfuric acid (e.g., magnesium sulfate), and other ions can be effectively added. If necessary, various vitamins such as thiamine (e.g., thiamine hydrochloride), amino acids such as glutamic acid (e.g., sodium glutamate) and asparagine (e.g., DL-asparagine), trace nutrients such as nucleotides, and selective drugs such as antibiotics can be added.
The pH of the medium is, for example, about 5.5 to 8. The cultivation can be carried out by a solid culture method under an aerobic condition, a shaking culture method, an agitation culture method with aeration, or an aerobic submerged culture method. In particular, an aerobic submerged culture method is most preferable. The culture temperature is appropriately 15° C. to 40° C., generally about 22° C. to 30° C. Although the production of the target substance varies depending on a medium, culture conditions, and a host used, the maximum accumulation can generally be attained in 2 to 10 days by any culture method. The incubation is terminated when the amount of the target substance in the medium reaches its peak, and the target substance is isolated from the culture and then purified.
In order to recover the target substance from the culture, an ordinary isolation method using its properties, such as a solvent extraction method, an ion-exchange resin method, an adsorption or distribution column chromatography method, a gel filtration method, a dialysis method, a precipitation method, and crystallization method, can be used singly or in appropriate combination for extraction and purification. For example, the substance is extracted from the culture with acetone, methanol, butanol, ethyl acetate, butyl acetate or the like.
For further purification of the target substance, chromatography using an adsorbent such as silica gel and alumina, Sephadex LH-20 (Pharmacia), or Toyopearl HW-40 (Tosoh Co.) can be carried out.
EXAMPLE
The present invention is further illustrated by the following examples that are not intended as a limitation of the invention.
1. Isolation of Genomic DNA and Construction of Genomic Library
A frozen seed culture of Streptomyces mycarofaciens (ATCC 21454) was inoculated into 50 ml of S #14 medium (2% glucose, 1% polypeptone, 0.05% K2HPO4, 0.05% MgSO4.7H2O, 0.3% NaCl, pH 7.0), and cultured at 28° C. for 20 hours. The culture was filtered using a bottle top filter 0.22 μm (Corning), after which the cells on the filter were washed twice with 10 mM EDTA and then recovered. The cells thus obtained were frozen with liquid nitrogen and then smashed with a mortar and pestle. The genomic DNA was isolated from these smashed cells using an ISOPLANT (Nippon gene) according to the attached protocol.
The isolated genomic DNA was partially digested with Sau3AI and then the resulting terminals were dephosphorylated. This DNA fragment was ligated with SuperCosI (Stratagene Co.) which had been digested with BamHI and XbaI (only the XbaI site was dephosphorylated) to construct a recombinant cosmid vector. This recombinant cosmid vector was subjected to in vitro packaging using a Max Plax Packaging Extract (Epicenter Technologies) according to the attached protocol. Then, Escherichia coli XL1-Blue MR strain was infected with this recombinant phage and incubated on a plate to form colonies.
2. Construction of Probes
The following primers were prepared from the conservative region of the PKS gene.










KS-F:
5′-CGGTSAAGTCSAACATCGG-3′
(SEQ ID NO: 44)



 


KS-R:
5′-GCRATCTCRCCCTGCGARTG-3′
(SEQ ID NO: 45)






PCR was carried out using KS-F and KS-R and the genomic DNA as a template. The PCR was carried out using an ExTaq DNA polymerase (Takara Shuzo Co., Ltd.). The amplified DNA fragment was inserted into a pCR2. 1-TOPO plasmid vector using a TOPO TA Cloning Kit (Invitrogen) according to the attached protocol.
The inserted DNA fragment was sequenced using a DNA Sequencing Kit dRhodamine Terminator Cycle Sequencing Ready Reaction (Perkin-Elmer) and an ABI PRISM Genetic Analyzer (Perkin-Elmer) according to the attached protocol. In this way, the isolated DNA fragment was confirmed to be a part of the PKS gene.
3. Screening of Cosmid Library
The DNA fragment was amplified by PCR using the plasmid containing a part of the midecamycin PKS gene as a template and primers KS-F and KS-R and used as a probe for hybridization.
A Hybond N+ membrane (Amersham Pharmacia Biotech) was placed on a plate, on which colonies of the genomic library were formed, to blot with the colonies. This membrane was treated with an alkali and upon cell lysis, the recombinant cosmid DNA on the membrane was denatured into a single chain and adsorbed on the membrane. Positive clones on the membrane were detected using an ECL Direct Nucleic Acid Labeling and Detecting System (Amersham Pharmacia Biotech) according to the attached protocol. In this way, cosmid clones pCOMW1 (FERM BP-8168) and pCOMW2 (FERM BP-8169) containing a region homologous to the probe were isolated. A probe was newly constructed by PCR from the terminal sequence of partially analyzed pCOMW1 (FERM -BP-8168). Screening of the genomic library was carried out again using this probe to isolate pCOMW4 (FERM BP-8170).
4. Determination of Base Sequences
pCOMW1 (FERM BP-8168) and pCOMW2 (FERM BP-8169) were partially digested with HaeIII, after which an about 2-kb fragment was purified by electrophoresis and ligated with pUC19 digested with SmaI. This plasmid was introduced into Escherichia coli XL1-Blue, the plasmid was extracted from a selected colony and was sequenced using −21M13 forward primer and M13 reverse primer as primers using an ABI3700 (Perkin-Elmer) according to the attached protocol. From the results obtained, regions where the analysis was not sufficient were further subjected to sequencing using primers newly designed based on already-analyzed base sequences. Further based on the results of this analysis, partial sequences of pCOMW4 (FERM BP-8170) were determined by primer walking. The positions of each cosmid clone are shown in FIG. 2.
5. Analysis of Nucleotide Sequences
Projection of ORFs was carried out using frame analysis attached to Genetyx (Software Development) and the functions of each ORF were projected by searching public databases using BLAST (Altschul, S. F. et al., J. Mol. Biol., 215, 403 (1990)). The positions of each ORF were shown in FIG. 3 and Table







TABLE 1







Positions of each ORF in SEQ ID NO: 1













Number of






amino
Bases in
Gene



SEQ ID NO:
acids
SEQ ID NO: 1
direction















ORF1
2
4511 
29244–42779
+


ORF2
3
1944 
42823–48657
+


ORF3
4
3696 
48712–59802
+


ORF4
5
1568 
59850–64556
+


ORF5
6
1892 
64687–70365
+


ORF6
7
237
70365–71078
+


ORF7
8
415
71113–72360
+


ORF8
9
421
72400–73665
+


ORF9
10
449
73694–75043
+


ORF10
11
223
75899–76570
−


ORF11
12
387
76602–77765
−


ORF12
13
424
78039–79313
+


ORF13
14
553
79391–81052
−


ORF14
15
271
81541–82356
+


ORF15
16
200
82760–83362
+


ORF16
17
215
83495–84142
−


ORF17
18
  (33)a
84329–84428
+


ORF18
19
348
27937–28983
+


ORF19
20
403
26180–27391
−


ORF20
21
152
25647–26105
−


ORF21
22
396
24460–25650
−


ORF22
23
302
23555–24463
−


ORF23
24
345
22534–23571
−


ORF24
25
264
21733–22527
−


ORF25
26
478
20307–21743
−


ORF26
27
388
19063–20229
+


ORF27
28
457
17522–18895
−


ORF28
29
607
15643–17466
+


ORF29
30
340
14074–15096
−


ORF30
31
342
13016–14044
−


ORF31
32
410
11729–12961
+


ORF32
33
360
10521–11603
+


ORF33
34
376
 9328–10458
+


ORF34
35
107
9012–9335
+


ORF35
36
288
8149–9015
+


ORF36
37
430
6653–7945
−


ORF37
38
193
6048–6629
−


ORF38
39
417
4695–5948
−


ORF39
40
484
3237–4691
−


ORF40
41
331
2220–3215
−


ORF41
42
344
1168–2202
−


ORF42
43
 (225)a
 1–675
−





aThe numbers set forth in the parentheses are indicated for partial sequences.






Further, functions inferred from each ORF are shown in Table 2.







TABLE 2







Inferred functions of each ORF














SEQ








ID


GenBank
Homology



NO
Highly homologous protein
Organism
No.
(%)
Function
















ORF1
2
Ty lactone synthase starter
Streptomyces
U78289
49
Polyketide synthase,




module, module 1, 2 TylG1
fradiae


macrolide skeleton synthesis


ORF2
3
Polyketide synthase module 3
Streptomyces
AF016585
60
Polyketide synthase,





caelestis


macrolide skeleton synthesis


ORF3
4
Ty lactone synthase module 4, 5
Streptomyces
U78289
59
Polyketide synthase,




TylGIII
fradiae


macrolide skeleton synthesis


ORF4
5
Polyketide synthase module 6
Streptomyces
AF016585
67
Polyketide synthase,





karestis


macrolide skeleton synthesis


ORF5
6
Polyketide synthase module 7
Streptomyces
AF016585
64
Polyketide synthase,





karestis


macrolide skeleton synthesis


ORF6
7
N-methyltransferase TylMI
Streptomyces
X81885
61
N-methyl transferase,





fradiae


mycaminose synthesis


ORF7
8
dnrQ
Streptomyces
L47164
37
NDP-hexose 3,4-isomerase,





neucetis


mycaminose synthesis


ORF8
9
Glycosyltransferase TylMII
Streptomyces
X81885
55
Glycosyltransferase,





fradiae


mycaminose addition


ORF9
10
Crotonyl-CoA reductase
Streptomyces
AL035161
80
Crotonyl-CoA reductase,





coelicolor


polyketide precursor








(ethylmalonyl-CoA) synthesis








polyketide precursor


ORF10
11
O-methyltransferase mdmC
Streptomyces
M93958
100 
O-methyltransferase,





mycarofaciens


polyketide presursor








(methoxymalonyl-ACP)








synthesis


ORF11
12
3-O-acyltrasnferase mdmB
Streptomyces
M93958
100 
3-O-acyltransferase,





mycarofaciens


macrolide skeleton








modification


ORF12
13
Cytochrome P-450
Streptomyces
D30759
64
Cytochrome P-450





thermotolerans


ORF13
14
Carbomycin resistance protein
Streptomyces
M80346
77
Midecamycin resistance





thermotolerans


protein


ORF14
15
Midecamycin tolerance protein
Streptomyces
A60725
100 
Midecamycin resistance




mdmA
mycarofaciens


protein


ORF15
16
TetR family transcription
Streptomyces
AL133220
49
TetR family transcription




control factor
coelicolor


control factor


ORP16
17
Unknown


—
Unknown


ORF17
18
4-Caoboxymuconolactone
Streptomyces
AL031155
 (67)a
4-Carboxymuconolactone




decarboxylase
coelicolor


decarboxylase


ORF18
19
Reductase
Streptomyces
AL355752
39
9-Reductase, macrolide





coelicolor


skeleton modification


ORF19
20
Cytochrome P-450 TylI
Streptomyces
U08223
64
19-Oxygenase, macrolide





fradiae


skeleton modification


ORF20
21
ORF15 × 4
Listonella
AF025396
39
Unknown





anguillarum


ORF21
22
Aminotransferase-like protein
Streptomyces
AF237895
61
Aminotransferase, mycaminose





antibioticus


synthesis


ORF22
23
α-D-Glucose-1-phosphate
Streptomyces
AF079762
69
α-D-Glucose-1-phosphate




thymidyltransferase
venezuelae


thymidyltransferase, deoxy








sugar synthesis


ORF23
24
AprE
Streptomyces
AF306787
69
dTDP-glucose 4,6-dehydratase,





tenebrareus


deoxy sugar synthesis


ORF24
25
RifR
Amycolatopsis
AF040570
50
Type II thioesterase,





mediterranei


macrolide skeleton








modification


ORF25
26
TDP-6-deoxy-4-ketohexose
Streptomyces
A7210634
54
TDP-6-deoxy-4-ketohexose




2,3-dehydratase
fradiae


2,3-dehydratase, mycarose








synthesis


ORF26
27
Midecamycin
Streptomyces
D63662
97
Midecamycin




4′′-O-propionyltransferase
mycarofaciens


4′′-O-propionyltransferase,








mycarose modification


ORF27
28
Control protein AcyB2
Streptomyces
D31821
55
TylR family transcription





thermotolerans


control factor


ORF28
29
SrmR
Streptomyces
X63451
76
SrmR family transcription





ambofaciens


control factor


ORF29
30
NDP-hexose 4-ketoreductase
Streptomyces
AF147704
55
NDP-hexose 4-ketoreductase,




TylCIV
fradiae


mycarose synthesis


ORF30
31
dTDP-keto-L-6-deoxy-hexose
Saccharoporis
U77454
73
dTDP-4-keto-L-6-deoxy-hexose




2,3-reductase
polaerislae


2,3-reductase, mycarose








synthesis


ORF31
32
NDP-hexose-3-C-methyltrans-
Streptomyces
AF147704
78
NDP-hexose-3-C-methyltrans-




ferase TylCIII
fradiae


ferase, mycarose synthesis


ORF32
33
FkbH
Streptomyces
A7235504
66
Glyceryl-ACP biosynthesis,





hygroscopicus


polyketide precursor








(methoxymalonyl-ACP)








synthesis


ORF33
34
FkbI
Streptomyces
AF235504
65
Acyl-CoA dehydrogenase,





hygroscopicus


polyketide precursor








(methoxymalonyl-ACP)








synthesis


ORF34
35
FkbJ
Streptomyces
AF235504
47
Acyl carrier protein,





hygroscopicus


polyketide precursor








(methoxymalonyl-ACP)








synthesis


ORF35
36
FkbK
Streptomyces
AF235504
56
3-Hydroxybutyril-CoA





hygroscopicus


dehydrogenase, polyketide








precursor








(methoxymalonyl-ACP)








synthesis


ORF36
37
Mycarosyltransferase TylCV
Streptomyces
AP147704
61
Glycosyltransferase, mycarose





fradiae


addition


ORF37
38
NDP-hexose-3,5-epimerase
Streptomyces
AF147704
74
NDP-hexose-3,5-epimerase,




TylCII
fradiae


mycarose synthesis


ORF38
39
Dehydratase
Streptomyces
AF055579
66
Dehydratase, desosamine





antibioticus


synthesis


ORF39
40
Reductase
Streptomyces
AF079762
69
Reductase, desosamine





venezuelae


synthesis


ORF40
41
Pyruvate dehydrogenase α
Coquella
AF387640
38
Pyruvate dehydrogenase α




subunit
varneddi


subunit


ORF41
42
Pyruvate dehydrogenase β
Sulfolobus
AE006767
42
Pyruvate dehydrogenase β




subunit
solfataricus


subunit


ORF42
43
Protein SC4H2.17
Streptomyces
AL022268
 (76)a
GTP-binding protein





coelicolor





aThe numbers set forth in the parentheses are indicated for partial sequences.






Further, biosynthesis pathways of midecamycins specified by functions are shown in FIGS. 4, 5, 6, and 7.
Genes encoding deoxysugar biosynthesis enzymes have been reported for erythromycin and tylosin (Summers, R. G. et al., Microbiology, 143, 3251 (1997); Gaisser, S. et al., Mol. Gen. Genet., 256, 239 (1997); Merson-Davies, L. A. and Cundliffe, E., Mol. Microbiol., 13, 349 (1994)). Syntheses of these deoxysugars include a step of glucose activation by addition of nucleotide diphosphate and a subsequent reaction such as dehydration, reduction, epimerization, amination, and methylation. These sugars are introduced into macrolides by action of specific glycosyltransferases.
The present inventors have identified the midecamycin biosynthesis pathway based on the structure of tylosin. The midecamycin biosynthesis starts with the syntheses of precursors of the polyketide skeleton, i.e., malonyl-CoA, methylmalonyl-CoA, ethylmalonyl-CoA, and methoxymalonyl-CoA. These precursors undergo stepwise condensation reactions and form rings, thereby polyketide skeletons being eventually synthesized, by polyketide synthesizing enzymes. After a series of modification reactions such as sugar chain addition, hydroxylation, formylation, and acylation, midecamycins are finally synthesized.
As for methoxymalonyl-ACP, which is a polyketide skeleton precursor of midecamycin, all the genes necessary for its biosynthesis (Wu, K. et al., Gene, 251, 81 (2000)) were present (FIG. 4). As for ethylmalonyl-CoA, ORF9 (crotonyl-CoA reductase) was applicable to its biosynthesis system but other genes were not found (FIG. 4).
ORF1 through ORF5 (PKS) and ORF24 (type II thioesterase) were considered to be involved in the biosynthesis of midecamycin polyketide skeletons (FIG. 5). Positions of modules and domains in ORF1 through ORF5 are shown in FIG. 8 and Tables 3, 4, 5, 6, and 7.







TABLE 3







Positions of each domain in ORF1












Bases of
Amino acids of



Domain
SEQ ID N0: 1
SEQ ID NO: 2







KSOnulla
29292–30509
 17–422



ATO
30813–31877
524–878



ACP0
31998–32255
 919–1004



KS1
32334–33611
1031–1456



AT1
33927–34991
1562–1916



KR1
35724–36590
2161–2449



ACP1
36666–36923
2475–2560



KS2
36990–38267
2583–3008



AT2
38628–39692
3129–3483



DH2
39738–40340
3499–3699



KR2
41307–42188
4022–4315



ACP2
42240–42497
4333–4418







aloss of function













TABLE 4







Positions of each domain in ORF2












Bases of
Amino acids of



Domain
SEQ ID N0: 1
SEQ ID NO: 3







KS3
42925–44202
35–460



AT3
44551–45609
577–929 



DH3
45649–46329
943–1169



KR3
47191–48054
1457–1744 



ACP3
48097–48354
1759–1844 

















TABLE 5







Positions of each domain in ORF3












Bases of
Amino acids of



Domain
SEQ ID NO: 1
SEQ ID NO: 4







KS4
48835–50112
 42–467



AT4
50413–51459
568–916



KR4nulla
52120–52935
1137–1408



ACP4
52960–53217
1417–1502



KS5
53275–54555
1522–1948



AT5
54901–55953
2064–2414



DH5
55987–56565
2426–2618



ER5
57256–58398
2939–3229



KR5
58366–59223
3219–3504



ACP5
59269–59526
3520–3605







aloss of function













TABLE 6







Positions of each domain in ORF4












Bases of
Amino acids of



Domain
SEQ ID N0: 1
SEQ ID NO: 5







KS6
59949–61223
 34–458



AT6
61536–62591
 563–914



KR6
63249–64103
 1134–1418



ACP6
64128–64376
 1427–1509

















TABLE 7







Positions of each domain in ORF5












Bases of
Amino acids of



Domain
SEQ ID NO: 1
SEQ ID NO: 6







KS7
64789–66066
 35–460



AT7
66412–67473
576–929



KR7
68335–69186
1217–1500



ACP7
69196–69459
1504–1591



TE7
69448–70362
1588–1892










A dysfunctional KS region that is commonly characteristic to PKS genes of 16-membered ring macrolide compounds was present near the N-terminal of ORF 1 of the midecamycin PKS gene (Table 3, FIG. 8). This is because C in the highly conserved region TVDTGCSSSLV (SEQ ID NO: 46) is substituted with Q (Aparicio, J. F. et al., Gene, 169, 9 (1996)).
KR in module 4 of ORF3 was also inferred to be dysfunctional (Table 5, FIG. 8). This is because the conservative region GXGXXGXXXA (SEQ ID NO: 47) in the KR is changed to DXTXXPXXXV (SEQ ID NO: 48) (Kakavas, S. J. et al., J. Bacteriol., 179, 7515 (1997)).
As for mycarose and mycaminose biosynthesis pathways, all the genes from glucose-1-phosphate to dTDP-mycarose and dTDP-mycaminose were present (FIG. 6).
As for genes involved in modification of midecamycin polyketide skeletons, all the genes which are involved in the binding of mycarose and mycaminose to the polyketide skeletons, such as genes for glycosyltransferase (ORF8, ORF36), acyltransferases for position 3 and position 4′′(ORF1l, ORF26), reductase for position 9 (ORF18), and position 19 oxygenase (ORF19), were present.
6. Confirmation of Functions
In order to confirm functions of each ORF of the isolated DNA fragment, homologous recombination is induced by incorporating a vector containing an internal fragment of each ORF or a vector in which a selectable marker gene is inserted dividing the internal part of each ORF, and thus a strain having the ORF disruption is constructed. A midecamycin intermediate produced when this gene disruption strain is cultured is extracted from the culture fluid with an appropriate organic solvent and the extract is analyzed using an LC-MS or the like to confirm functions of each ORF (Wilson, V. T. W. and Cundliffe, E., Gene, 214, 95 (1998); Butler, A. R. et al., Chem. Biol., 6, 287 (1999); Kakavas, S. J. et al., J. Bacteriol., 179, 7515 (1997)). Further, each ORF is ligated with a vector having an appropriate promoter and a terminator for expression and the vector is introduced into a host microorganism other than Streptomyces mycarofaciens. Functions of each ORF are confirmed by producing a compound by adding a substrate inferred from the ORF introduced upon cultivation of this recombinant or by utilizing an endogenous substrate of the host microorganism by extracting the produced compound with an appropriate organic solvent from the culture fluid, and then by analyzing the extract using an LC-MS or the like (Hara, O. and Hutchinson, C. R., J. Antibiot., 43, 977 (1990); Hara, O. and Hutchinson, C. R., J. Bacteriol., 174, 5141 (1992)).

## Claims

1. An isolated polynucleotide comprising a nucleotide sequence encoding a protein which is involved in midecamycin biosynthesis, wherein said protein comprises an amino acid sequence selected from the group consisting of the following sequences:
(a) the amino acid sequence of SEQ ID NO: 3,
(b) the amino acid sequence of ORF2 protein, which is encoded by a clone contained in the microorganism deposited under an accession number of FERM BP-8168, and
(c) a modified amino acid sequence of (a) or (b) having one to 40 conservative amino acid modifications wherein said modified amino acid sequence has polyketide synthase activity.

2. The polynucleotide according to claim 1, which comprises the nucleotide sequence of bases 42823–48657, of SEQ ID NO: 1.

3. An isolated polynucleotide comprising a nucleotide sequence encoding a functional domain of polyketide synthase (PKS) which is involved in midecamycin biosynthesis, wherein said domain comprises an amino acid sequence selected from the group consisting of the following sequences:
(1) an amino acid sequence selected from amino acid residues 35–460, 577–929, 943–1169, 1457–1744, and 1759–1844 of SEQ ID NO: 3,
(2) the amino acid sequence of a functional domain of ORF2 protein, which is encoded by a clone contained in the microorganism deposited under an accession number of FERM BP-8168 and
(3) an amino acid sequence of either (1) or (2) having one to eight conservative amino acid modifications wherein said modified amino acid sequence has polyketide synthase activity.

4. The polynucleotide according to claim 3, which comprises a nucleotide sequence selected from bases 42925–44202, 44551–45609, 45649–46329, 47191–48054, and 48097–48354 of SEQ ID NO: 1.

5. An isolated polynucleotide comprising a nucleotide sequence encoding a protein which is involved in midecamycin biosynthesis, wherein said nucleotide sequence comprises a nucleotide sequence which can hybridize with a nucleotide sequence which encodes the amino acid sequence of SEQ ID NO: 3, under stringent conditions comprising 0.2×SSC in a 0.1% SDS solution at 60° C. for 15 minutes.

6. An isolated polynucleotide comprising a nucleotide sequence encoding a functional domain of polyketide synthase (PKS) which is involved in midecamycin biosynthesis, wherein said nucleotide sequence is a nucleotide sequence which can hybridize with a nucleotide sequence encoding an amino acid sequence selected from amino acid residues 35–460, 577–929, 943–1169, 1457–1744, and 1759–1844 of SEQ ID NO: 3, under stringent conditions
wherein said stringent conditions comprise 0.2×SSC in a 0.1% SDS solution at 60° C. for 15 minutes.

7. A recombinant vector comprising the polynucleotide of claim 1.

8. A recombinant vector comprising the polynucleotide of claim 3.

9. A recombinant vector comprising the polynucleotide of claim 5.

10. A recombinant vector comprising the polynucleotide of claim 6.

11. A host cell comprising the recombinant vector of claim 7.

12. A host cell comprising the recombinant vector of claim 8.

13. A host cell comprising the recombinant vector of claim 9.

14. A host cell comprising the recombinant vector of claim 10.