Patent Publication Number: US-2006019301-A1

Title: Methods of producing mutant polynucleotides

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of U.S. provisional application Ser. No. 60/589,502 filed on Jul. 20, 2004, and U.S. provisional application Ser. No. 60/633,756 filed on Dec. 6, 2004, which applications are fully incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to methods of producing mutants of a polynucleotide and to mutant polynucleotides and artificial variants encoded by the mutant polynucleotides.  
      2. Description of the Related Art  
      The diversity necessary for screening in directed evolution of proteins is often created by error prone mutagenesis to find mutations or positions influencing enzyme activity. Although error prone mutagenesis in principle mutates all base pairs randomly, the outcome of the mutagenesis is rather limited for two main reasons: (A) a given amino acid codon is typically mutated to only 6 or 7 other residues (from one substitution per codon, two or three substitutions are very unlikely), and (B) the mutation rate is biased towards A-T base pairs. Typically 75% of the mutated base pairs are A-T pairs, leaving only 25% of mutated G-C pairs, and the resulting mutation is also biased towards certain bases. Also, additional mutations are normally included to overcome silent mutations, which enhance the chance of hitting destructive mutations due to error in folding, maturation, secretion, etc.  
      Transposons are segments of DNA that can move around to different positions in the genome of a single cell. They can cause mutations and/or an increase (or decrease) in the amount of DNA in the genome. These mobile segments of DNA are sometimes called “jumping genes”.  
      Many transposons move by a “cut and paste” process. The transposon is cut out of its location and inserted into a new location. This process requires a transposase that is encoded within some transposons. Transposase binds to both ends of the transposon, which consists of inverted repeats which are identical sequences reading in opposite directions, and to a sequence of DNA that makes up the target site. Some transposases require a specific sequence as their target site while others can insert the transposon anywhere in the genome. The DNA at the target site is cut in an offset manner, like the “sticky ends” produced by some restriction enzymes. After the transposon is ligated to the host DNA, the gaps are filled in by Watson-Crick base pairing, which creates identical direct repeats at each end of the transposon.  
      Often transposons lose their gene for transposase, but as long as there is a transposon in the cell that can synthesize the enzyme, their inverted repeats are recognized and they, too, can be moved to a new location. Alternatively, if it desirable that the transposon remains stably integrated in the same place, the transposase may be provided transiently in trans, which is often the case when in vitro transposition is carried out.  
      Transposons have proven to be invaluable genetic tools for molecular geneticists. Several uses of transposons include mutagenesis for gene identification, reporter libraries for analysis of gene expression, and DNA sequencing for relative gene positioning on genetic maps. Until recently, however, all of these applications involved the use of in vivo transposition reactions. However, the commercialization of several in vitro transposition reactions for DNA sequencing and mutagenesis could lead to the replacement of these more traditional in vivo methodologies with more efficient biochemical procedures.  
      The use of in vitro transposition for the mutagenesis of specific genes was first reported by Gwinn et al., 1997 , Journal of Bacteriology  179: 7315-7320, where genomic DNA from a naturally transformable microorganism ( Haemophilus influenzae ) was mutagenized using the Tn7 in vitro transposition system. DNA sequencing using primers that hybridize to the end of the transposon identified mutations in the genes resulting in a reduced expression of constitutive competence genes.  
      Reich et al., 1999 , Journal of Bacteriology  181: 4961-4968, disclose the use of the Ty1-based transposition system (Primer Island) to scan the entire  Haemophilus influenzae  genome for essential genes. Essential genes were identified by two methods: mutation exclusion and zero time analysis. Mutational exclusion involves the identification of open reading frames that do not contain transposon insertions. Zero time analysis involves the monitoring of the growth of individual cells after transformations over time.  
      U.S. Pat. No. 6,673,567 discloses methods for identifying genes, open reading frames, and other nucleic acid molecules which are essential for the expression of a specific phenotype in microorganisms. The method employs in vitro transposition in conjunction with a chromosomal integration vector containing a specific gene or genetic element whose function is unknown. Subsequent transformation of a recombination proficient host with the vector and growth first under non-integrating conditions and then under integrating conditions, followed by a selection screen for either single or double crossover events, results in transformants that may be subjected to phenotypic screens to determine gene function.  
      U.S. Pat. No. 6,562,624 discloses methods for facilitating site-directed homologous recombination in a eukaryotic organism to produce genomic mutants using transposon-mediated mutagenesis of cosmid vectors carrying large genomic inserts from the target eukaryotic organism. The transposon carries a bifunctional marker that can be used for selection in both bacteria and the target eukaryotic organism. Minimization of the length of the cosmid vector allows for maximization of the size of the genomic insert carried by the cosmid. Maximization of the size of the genomic insert increases the frequency of homologous recombination with the genome of the target eukaryotic organism.  
      The present transposon-based mutagenesis technology is limited in its application because there is no differentiation between mutants in which a transposon has inserted into target DNA versus mutants that have the transposon inserted into adjacent, non-target DNA such as plasmid vector sequences. Previously, to create a mutagenic library that contained only clones in which the transposon was targeted to the desired DNA sequence required excision, purification, and subcloning of those target DNA&#39;s containing a transposon. There is a need in the art for a simplified method of subcloning transposon-containing targeted DNA in a single step.  
      Applying transposon technology combined with outside cutters (restriction endonucleases cutting outside their recognition sequence), it is possible to produce a polypeptide library with one or more substituted amino acids. For instance, an amino acid in a position may be substituted to provide a polypeptide library including each of the remaining 20 natural amino acids in that position. Applying transposon technology and outsite cutters, it is also possible to produce polypeptide libraries with insertions or deletions: in theory any number of coding triplets can be inserted, and with the outside cutters presently known up to 5 triplets can be deleted, but this number may increase with the discovery of new outside cutters that cut farther away from their recognition sequence than the ones presently known.  
      The object of the present invention is to provide new methods of producing mutant polynucleotides.  
     SUMMARY OF THE INVENTION  
      The present invention relates to methods of producing at least one mutant of a polynucleotide, the method comprising the steps of: 
          (a) isolating a first library of constructs, wherein each construct comprises a first selectable marker, a polynucleotide, an inserted artificial transposon comprising at least two restriction endonuclease recognition sites and a second selectable marker, and a first recombination site flanking the 5′ end of the polynucleotide and a second recombination site flanking the 3′ end of the polynucleotide, wherein the artificial transposon has inserted at one or more random sites within the constructs, and wherein the first library is selected using the first and second selectable markers in a first host cell;     (b) isolating a second library of constructs by introducing the first library of constructs into a vector comprising a third selectable marker and a first recombination site and a second recombination site to facilitate site-specific recombination of the first recombination site flanking the 5′ end of the polynucleotide and the second recombination site flanking the 3′ end of the polynucleotide in the first library of constructs with the first recombination site and the second recombination site of the vector and by selecting the second library of constructs using the second and third selectable markers in a second host cell;     (c) isolating an insertion library containing at least one substitution, deletion, or insertion of at least one nucleotide in each polynucleotide of the second library of constructs by removing all, essentially all, or a portion of the inserted artificial transposon from the second library of constructs through restriction endonuclease digestion of the at least two restriction endonuclease recognition sites leaving at least one substitution, deletion, or insertion of at least one nucleotide in the polynucleotide; self-ligating the restriction endonuclease digested fragments; and selecting the insertion library using the third selection marker in a third host cell; and     (d) isolating at least one mutant of the polynucleotide from the insertion library, wherein the isolated mutant comprises at least one substitution, deletion, or insertion of at least one nucleotide in the polynucleotide.        

      The present invention also relates to methods of producing at least one polynucleotide encoding at least one variant of a parent polypeptide, the method comprising the steps of: 
          (a) providing a nucleic acid construct comprising a polynucleotide encoding the parent polypeptide, into which polynucleotide has been inserted a heterologous polynucleotide fragment, wherein said fragment comprises at least two restriction endonuclease recognition sites;     (b) restricting the nucleic acid construct with at least two corresponding restriction endonucleases, if necessary in separate individual steps of restricting, PCR-polishing, and ligating, wherein all or essentially all of the inserted heterologous fragment is excised from the construct and at least one nucleotide triplet is deleted, inserted, or substituted in the encoding polynucleotide in the process, whereby at least one polynucleotide encoding at least one variant of the parent polypeptide is produced.        

      The present invention also relates to polynucleotide constructs comprising a transposon, said transposon comprising one or more outside cutter restriction endonuclease recognition sites.  
      The present invention also relates to cells comprising in its genome an integrated heterologous polynucleotide fragment, said fragment comprising one or more outside cutter restriction endonuclease recognition sites.  
      The present invention also relates to isolated mutant polynucleotides obtained by such methods; nucleic acid constructs, expression vectors, and host cells comprising such mutant polynucleotides; and methods for producing artificial variants of a polypeptide encoded by such mutant polynucleotides. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       FIG. 1  shows a restriction map of pSATe101.  
       FIG. 2  shows a restriction map of pSATe111.  
       FIG. 3  shows a restriction map of pAJF-1.  
       FIG. 4  shows a restriction map of pAJF-2.  
       FIG. 5  shows the distribution of transposon insertions of an  Aspergillus oryzae  beta-glucosidase gene based on the sequences of 50 clones.  
       FIG. 6  shows phenotype distribution based on transposon insertion position of the  Aspergillus oryzae  beta-glucosidase gene. Each box covering the clone numbers denotes a specific phenotype observed for that clone using a X-glc colorimetric plate assay for beta-glucosidase activity.  
       FIG. 7A  shows two oligonucleotide primers (SEQ ID NO: 7 and SEQ ID NO: 8) designed to PCR-amplify a DNA-fragment suitable to be cloned into the flanking Not I-sites of a transposon already inserted in a gene of interest, using the transposon shown in SEQ ID NO: 9 as PCR template. The complementary primer sequences are shown in grey typeface. The primers and consequently also the DNA-fragment comprise a number of restriction endonuclease enzyme recognition sites that are indicated as underlined and/or italicized nucleotides in the sequences, the corresponding enzymes are noted above and below the sequences. In addition, the fragment comprises a random or partially random codon triplet ‘NNN’.  
       FIG. 7B  shows ends of the the PCR-fragment after it has been cloned into the transposon in the gene of interest, thus replacing the transposon. The nucleotides in bold typeface, the X&#39;es, and nucleotides 1 through 5, are part of the gene of interest, whereas the normal font nucleotides represent heterologous DNA which has been inserted into the gene. The nucleotides marked 1 through 5 serve to illustrate the target site in the gene of interest where the random or partially random codon triplet ‘NNN’ will finally be located in the resulting polynucleotide sequence. It is shown that the target site is duplicated by the insertion of the transposon. The full sequence of the transposon with the PCR-fragment cloned into the Not I sites is shown in SEQ ID NO: 10.  
       FIG. 7C  shows how the DNA-fragment has been designed, so that the restriction in C with the outside cutter enzyme Bsg I, followed by a PCR-polishing to remove any nucleotide overhangs in the resulting fragments, will bring the right-hand side of the random or partially random triple codon ‘NNN’ into position directly adjacent to the nucleotides of the gene of interest (shown in bold typeface) after a ligation step.  
       FIG. 7D  shows how the DNA-fragment has been designed, so that restriction with the outside cutter enzyme Btg ZI in combination with the enzyme Pvu II, followed by a PCR-polishing filling in of the overhanging nucleotides in the resulting fragments, will bring the DNA-fragment into a suitable position directly adjacent to the nucleotides ‘1’ and ‘2’ of the target site in the gene of interest (in bold typeface) after a ligation step.  
       FIG. 7E  shows the final restriction with the outside cutter enzyme Bfu AI, whereby the entire remaining inserted heterologous DNA-fragment is removed from the gene of interest, leaving behind only an overhang of the random or partially random triple codon ‘NNN’, which after a subsequent PCR-polishing and a ligation step produces a resulting polynucleotide, wherein the nucleotide triplet consisting of nucleotides ‘3’, ‘4’, and ‘5’ of the target polynucleotide has been replaced with the random or partially random triplet codon denoted by ‘NNN’.  
       FIG. 8A  shows two oligonucleotide primers (SEQ ID NO: 11 and SEQ ID NO: 12) designed to PCR-amplify a DNA-fragment suitable to be cloned into the flanking Not I-sites of a transposon already inserted in a gene of interest, using the transposon shown in SEQ ID NO: 9 as PCR template. The complementary primer sequences are shown in grey typeface. The primers and consequently also the DNA-fragment comprise a number of restriction endonuclease enzyme recognition sites that are indicated as underlined and/or italicized nucleotides in the sequences, the corresponding enzymes are noted above and below the sequences. In addition, the fragment comprises a random or partially random codon triplet ‘NNN’.  
       FIG. 8B  shows the ends of the PCR-fragment after it has been cloned into the transposon in the gene of interest, thus replacing the transposon. The nucleotides in bold typeface, the X&#39;es, and nucleotides 1 through 5, are part of the gene of interest, whereas the normal font nucleotides represent heterologous DNA which has been inserted into the gene. The nucleotides marked 1 through 5 serve to illustrate the target site in the gene of interest where the random or partially random codon triplet ‘NNN’ will finally be located in the resulting polynucleotide sequence. It is shown that the target site is duplicated by the insertion of the transposon. The full sequence of the transposon with the PCR-fragment cloned into the Not I sites is shown in SEQ ID NO: 13.  
       FIG. 8C  shows how the DNA-fragment of has been designed, so that the restriction in C with the outside cutter enzyme Bsg I, followed by a PCR-polishing to remove any nucleotide overhangs in the resulting fragments, will bring the right-hand side of the random or partially random triple codon ‘NNN’ into position directly adjacent to the nucleotides of the gene of interest (shown in bold typeface) after a ligation step.  
       FIG. 8D  shows the final restriction with the outside cutter enzyme Acu I, whereby the entire remaining inserted heterologous DNA-fragment is removed from the gene of interest, leaving behind only an overhang of the random or partially random triple codon ‘NNN’, which after a subsequent PCR-polishing and a ligation step produces a resulting polynucleotide, wherein the nucleotide triplet consisting of nucleotides ‘3’, ‘4’, and ‘5’ of the target polynucleotide has been replaced with the random or partially random triplet codon denoted by ‘NNN’.  
       FIG. 9A  shows two oligonucleotide primers (SEQ ID NO: 14 and SEQ ID NO: 15) designed to PCR-amplify a DNA-fragment suitable to be cloned into the flanking Not I-sites of a transposon already inserted in a gene of interest, using the transposon shown in SEQ ID NO: 9 as PCR template. The complementary primer sequences are shown in grey typeface. The primers and consequently also the DNA-fragment comprise a number of restriction endonuclease enzyme recognition sites that are indicated as underlined and/or italicized nucleotides in the sequences, the corresponding enzymes are noted above and below the sequences.  
       FIG. 9B  shows the ends of the PCR-fragment after it has been cloned into the transposon in the gene of interest, thus replacing the transposon. The nucleotides in bold typeface, the X&#39;es, and nucleotides 1 through 5 on the left side, and 1 through 8 on the right side, are part of the gene of interest, whereas the normal font nucleotides represent heterologous DNA which has been inserted into the gene. The nucleotides marked 1 through 5 on the left side, and 1 through 7 on the right side, serve to illustrate the target site in the gene of interest where the deleted codon triplet will finally be “located” in the resulting polynucleotide sequence. It is shown that the target site is duplicated by the insertion of the transposon. The full sequence of the transposon with the PCR-fragment cloned into the Not I sites is shown in SEQ ID NO: 16.  
       FIG. 9C  shows restriction with the outside cutter enzyme Acu I, whereby the entire remaining inserted heterologous DNA-fragment is removed from the gene of interest, leaving behind only an overhang of the deleted codon triplet, which after a subsequent PCR-polishing and a ligation step produces a resulting polynucleotide, wherein the nucleotide triplet consisting of nucleotides ‘5’, ‘6’, and ‘7’ in the target polynucleotide has been deleted.  
      Definitions  
      Inside cutter: The term “inside cutter” or “inside cutting endonuclease” is defined herein as a restriction endonuclease which digests a DNA sequence inside the actual recognition sequence or site. By far the majority of restriction endonucleases belong to this group. Indeed a very large number of these enzymes are known, and have been known for decades, e.g. Eco RI or Bam HI.  
      Outside cutter: The term “outside cutter” or “outside cutting endonuclease” is defined herein as a restriction endonuclease which digests a DNA sequence outside the actual recognition sequence or site. These endonucleases, which are subclasses of Type II enzymes (Szybalski et al., 1991, Gene 100: 13-26), are commercially available from a number of vendors and listed in REBASE. Non-limiting examples of outside cutters are Aar I, Ace III, Alf I, Alo I, Bae I, Bbr 7I, Bbv I, Bbv II, Bcc I, Bce 83I, Bce AI, Bce fI, Bcg I, Bcl VI, Bfl I, Bin I, Bpl I, Bsa XI, Bsa XI, Bsc AI, Bse MII, Bse RI, Bsg I, Bsl FI, Bsm I, Bsm AI, Bsm FI, Bsp 24I, Bsp CNI, Bsp MI, Bsr I, Bsr DI, Bst F5I, Btg ZI, Bts I, Cha I, Cje I, Cje PI, Csp CI, Cst MI, and Eci I.  
      PCR polishing: The term “PCR polishing” refers to in vitro methods of blunt-ending nucleotide overhangs in a polynucleotide fragment after restriction by an endonuclease. Many restriction endonucleases leave behind either a 5′ or 3′ nucleotide overhang, the so-called “sticky ends”, and if two fragments have incompatible overhangs then they cannot be ligated together.  
      Isolated polynucleotides: The term “isolated polynucleotide” or “isolated mutant polynucleotide” as used herein refers to a polynucleotide which is at least 20% pure, preferably at least 40% pure, more preferably at least 60% pure, even more preferably at least 80% pure, most preferably at least 90% pure, and even most preferably at least 95% pure, as determined by agarose electrophoresis.  
      Substantially pure polynucleotides: The term “substantially pure polynucleotide” or “substantially pure mutant polynucleotide” as used herein refers to a polynucleotide preparation free of other extraneous or unwanted nucleotides and in a form suitable for use within genetically engineered production systems. Thus, such substantially pure polynucleotides contain at most 10%, preferably at most 8%, more preferably at most 6%, more preferably at most 5%, more preferably at most 4%, more preferably at most 3%, even more preferably at most 2%, most preferably at most 1%, and even most preferably at most 0.5% by weight of other polynucleotide material with which it is natively or recombinantly associated. A substantially pure polynucleotide may, however, include naturally occurring 5′ and 3′ untranslated regions, such as promoters and terminators. It is preferred that the substantially pure polynucleotide is at least 90% pure, preferably at least 92% pure, more preferably at least 94% pure, more preferably at least 95% pure, more preferably at least 96% pure, more preferably at least 97% pure, even more preferably at least 98% pure, most preferably at least 99%, and even most preferably at least 99.5% pure by weight. The polynucleotides of the present invention are preferably in a substantially pure form. In particular, it is preferred that the polynucleotides disclosed herein are in “essentially pure form”, i.e., that the polynucleotide preparation is essentially free of other polynucleotide material with which it is natively or recombinantly associated. Herein, the term “substantially pure polynucleotide” is synonymous with the terms “isolated polynucleotide” and “polynucleotide in isolated form.” The polynucleotides may be of genomic, cDNA, RNA, semisynthetic, synthetic origin, or any combinations thereof.  
      cDNA: The term “cDNA” is defined herein as a DNA molecule which can be prepared by reverse transcription from a mature, spliced, mRNA molecule obtained from a eukaryotic cell. cDNA lacks intron sequences that are usually present in the corresponding genomic DNA. The initial, primary RNA transcript is a precursor to mRNA which is processed through a series of steps before appearing as mature spliced mRNA. These steps include the removal of intron sequences by a process called splicing. cDNA derived from mRNA lacks, therefore, any intron sequences.  
      Nucleic acid construct: The term “nucleic acid construct” or simply “construct” as used herein refers to a nucleic acid molecule, either single- or double-stranded, which is isolated from a naturally occurring gene or which is modified to contain segments of nucleic acids in a manner that would not otherwise exist in nature. The term nucleic acid construct is synonymous with the term “expression cassette” when the nucleic acid construct contains the control sequences required for expression of a coding sequence of the present invention.  
      Control sequence: The term “control sequences” is defined herein to include all components, which are necessary or advantageous for the expression of a polynucleotide encoding an artificial variant of a polypeptide. Each control sequence may be native or foreign to the nucleotide sequence encoding the polypeptide. Such control sequences include, but are not limited to, a leader, polyadenylation sequence, propeptide sequence, promoter, signal peptide sequence, and transcription terminator. At a minimum, the control sequences include a promoter, and transcriptional and translational stop signals. The control sequences may be provided with linkers for the purpose of introducing specific restriction sites facilitating ligation of the control sequences with the coding region of the nucleotide sequence encoding a polypeptide.  
      Operably linked: The term “operably linked” denotes herein a configuration in which a control sequence is placed at an appropriate position relative to the coding sequence of the polynucleotide sequence such that the control sequence directs the expression of the coding sequence of a polypeptide.  
      Coding sequence: When used herein the term “coding sequence” means a nucleotide sequence, which directly specifies the amino acid sequence of its protein product. The boundaries of the coding sequence are generally determined by an open reading frame, which usually begins with the ATG start codon or alternative start codons such as GTG and TTG. The coding sequence may a DNA, cDNA, or recombinant nucleotide sequence.  
      Expression: The term “expression” includes any step involved in the production of the polypeptide including, but not limited to, transcription, post-transcriptional modification, translation, post-translational modification, and secretion.  
      Expression vector: The term “expression vector” is defined herein as a linear or circular DNA molecule that comprises a polynucleotide encoding a polypeptide, and which is operably linked to additional nucleotides that provide for its expression.  
      Host cell: The term “host cell”, as used herein, includes any cell type which is susceptible to transformation, transfection, transduction, and the like with a nucleic acid construct or expression vector comprising a polynucleotide.  
      Modification: The term “modification” or “modified polynucleotide” means herein any chemical modification as well as genetic manipulation of the DNA encoding that polypeptide. The modification can be substitutions, deletions and/or insertions of one or more amino acids as well as replacements of one or more amino acid side chains.  
      Parent polypeptide: The term “parent polypeptide” as used herein means a polypeptide to which modifications, e.g., substitution(s), insertion(s), deletion(s), and/or truncation(s), are made to produce artificial variants. This term also refers to the polypeptide with which a variant is compared and aligned. The parent may be a naturally occurring (wild type) polypeptide, or it may even be a variant thereof, prepared by any suitable means. For instance, the parent polypeptide may be a variant of a naturally occurring polypeptide which has been modified or altered in the amino acid sequence. A parent polypeptide may also be an allelic variant which is a polypeptide encoded by any of two or more alternative forms of a gene occupying the same chromosomal locus.  
      Artificial variant: When used herein, the term “artificial variant” means a polypeptide produced by an organism expressing a modified nucleotide sequence, where the modified nucleotide sequence is obtained through human intervention by modification of the nucleotide sequence.  
      Transposon and transposase: The term “transposon” is defined herein as a region of nucleic acid that is capable of moving from one position to another within DNA where this movement is catalyzed by a transposase. Transposons are also known as “transposable elements”.  
      Artificial transposon: When used herein, the term “artificial transposon” means a modified transposon obtained through human intervention by modification of the nucleotide sequence.  
      Transposase: The term “transposase” means a protein that catalyses the steps, i.e., breakage and joining, of a transposition reaction.  
      In vitro transposition: The term “in vitro transposition” is defined herein as a biochemical reaction that is initiated outside the cell that catalyzes the movement of a transposable element from one site into a different site within the same or a different DNA molecule.  
      In vivo transposition: The term “in vivo transposition” means a biochemical reaction that takes place within the cell that catalyzes the mobilization of a transposon from one site to another site within the genome of the host.  
      Recombinase: The term “recombinase” is defined herein as a ubiquitous class of enzymes which catalyze DNA strand recombination in bacteria, yeast,  Drosophila , immunoglobulin and T cell receptor gene rearrangement, and other systems. Site-specific recombinases include, but are not limited to, bacteriophage P1 Cre recombinase, yeast FLP recombinase, Inti integrase, bacteriophage lambda, phi 80, P22, P2, 186, and P4 recombinase, Tn3 resolvase, the Hin recombinase, the Cin recombinase,  E. coli  xerC and xerD recombinases,  Bacillus thuringiensis  recombinase, TpnI, the beta-lactamase transposons, and the immunoglobulin recombinases.  
      Recombination: The term “recombination” is defined herein as a process wherein nucleic acids associate with each other in regions of homology, leading to interstrand DNA exchange between those sequences. For purposes of the present invention, homologous recombination is determined according to the procedures summarized by Paques and Haber, 1999 , Microbiology and Molecular Biology Reviews  63: 349-404. “Homologous recombination” is defined herein as recombination in which no changes in the nucleotide sequences occur within the regions of homology relative to the input nucleotide sequences. For perfect homologous recombination, the regions should contain a sufficient number of nucleic acids, such as 15 to 1,500 base pairs, preferably 100 to 1,500 base pairs, more preferably 400 to 1,500 base pairs, and most preferably 800 to 1,500 base pairs, which are highly homologous with the corresponding nucleic acid sequence to enhance the probability of homologous recombination.  
      Improved property: The term “improved property” is defined herein as a characteristic associated with a mutant polynucleotide which is improved compared to the parent polynucleotide or a variant polypeptide encoded by a mutant polynucleotide which is improved compared to the parent polypeptide. Such improved properties include, but are not limited to, altered control sequence function, altered temperature-dependent activity profile, thermostability, pH activity, pH stability, substrate specificity, product specificity, and chemical stability.  
      Altered control sequence function: The term “altered control sequence function” is defined herein as an alteration of the endogenous function of a control sequence. This may include, but is not limited to, alterations which affect the level of transcription, the stability of the messenger RNA transcribed, the degree or type of messenger RNA processing, the level of secretion, the localization of the controlled protein, or proteolytic processing of the controlled protein.  
      Improved thermal activity: The term “improved thermal activity” is defined herein as an alteration of the temperature-dependent activity profile of a variant enzyme at a specific temperature relative to the temperature-dependent activity profile of the parent enzyme. The thermal activity value provides a measure of the enzyme&#39;s efficiency in performing catalysis of a reaction over a range of temperatures. An enzyme has a specific temperature range wherein the protein is stable and retains its enzymatic activity, but becomes less stable and thus less active with increasing temperature. Furthermore, the initial rate of a reaction catalyzed by an enzyme can be accelerated by an increase in temperature which is measured by determining thermal activity of a variant. A more thermoactive variant will lead to an increase in the rate of catalysis decreasing the time required and/or decreasing the enzyme concentration required for catalysis. Alternatively, a variant with a reduced thermal activity will catalyze a reaction at a temperature lower than the temperature optimum of the parent enzyme defined by the temperature-dependent activity profile of the parent.  
      Improved thermostability: The term “improved thermostability” is defined herein as a variant enzyme displaying retention of enzymatic activity after a period of incubation at elevated temperature relative to the parent enzyme. Such a variant may or may not display an altered thermal activity profile relative to the parent. For example, a variant may have an improved ability to refold following incubation at elevated temperature relative to the parent.  
      In a preferred embodiment, the thermal activity of the variant enzyme is at least 1.5-fold, preferably at least 2-fold, more preferably at least 5-fold, most preferably at least 7-fold, and even most preferably at least 20-fold more thermally active than the wild-type variant under specified conditions.  
      Improved product specificity: The term “improved product specificity” is defined herein as a variant enzyme displaying an altered product profile relative to the parent in which the altered product profile improves the performance of the variant in a given application relative to the parent. The term “product profile” is defined herein as the chemical composition of the reaction products produced by enzymatic catalysis.  
      Improved chemical stability: The term “improved chemical stability” is defined herein as a variant enzyme displaying retention of enzymatic activity after a period of incubation in the presence of a chemical or chemicals, either naturally occurring or synthetic, which reduce the enzymatic activity of the parent enzyme. Improved chemical stability may also result in variants better able to catalyze a reaction in the presence of such chemicals. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In a first aspect, the present invention relates to methods of producing at least one mutant of a polynucleotide, the method comprising the steps of: 
          (a) isolating a first library of constructs, wherein each construct comprises a first selectable marker, a polynucleotide, an inserted artificial transposon comprising at least two restriction endonuclease recognition sites and a second selectable marker, and a first recombination site flanking the 5′ end of the polynucleotide and a second recombination site flanking the 3′ end of the polynucleotide, wherein the artificial transposon has inserted at one or more random sites within the constructs, and wherein the first library is selected using the first and second selectable markers in a first host cell;     (b) isolating a second library of constructs by introducing the first library of constructs into a vector comprising a third selectable marker and a first recombination site and a second recombination site to facilitate site-specific recombination of the first recombination site flanking the 5′ end of the polynucleotide and the second recombination site flanking the 3′ end of the polynucleotide in the first library of constructs with the first recombination site and the second recombination site of the vector and by selecting the second library of constructs using the second and third selectable markers in a second host cell;     (c) isolating an insertion library containing at least one substitution, deletion, or insertion of at least one nucleotide in each polynucleotide of the second library of constructs by removing all, essentially all, or a portion of the inserted artificial transposon from the second library of constructs through restriction endonuclease digestion of the at least two restriction endonuclease recognition sites leaving at least one substitution, deletion, or insertion of at least one nucleotide in the polynucleotide; self-ligating the restriction endonuclease digested fragments; and selecting the insertion library using the third selection marker in a third host cell; and     (d) isolating at least one mutant of the polynucleotide from the insertion library, wherein the isolated mutant comprises at least one substitution, deletion, or insertion of at least one nucleotide in the polynucleotide.        

      First Library. In the methods of the present invention, a first library of constructs is isolated, wherein each construct comprises a first selectable marker, a polynucleotide, an inserted artificial transposon comprising at least two restriction endonuclease recognition sites and a second selectable marker, and a first recombination site flanking the 5′ end of the polynucleotide and a second recombination site flanking the 3′ end of the polynucleotide, wherein the artificial transposon has inserted at one or more random sites within the constructs, and wherein the first library is selected using the first and second selectable markers in a suitable host cell.  
      In a preferred aspect, the polynucleotide of interest is modified so it contains desired restriction sites to facilitate cloning of the polynucleotide into a vector, for example, an entry vector. PCR can be used in conjunction with specific primers to amplify the polynucleotide of interest to incorporate the desired restriction sites. In a preferred aspect, the polynucleotide of interest is blunt-ended using a thermostable, proofreading polymerase for directionally cloning the polynucleotide into a vector for the “first library of constructs”, e.g., an entry vector, and transformation of the vector into a suitable host, e.g.,  E. coli.    
      Any vector can be used in the methods of the present invention for the “first library of constructs”, e.g., entry vector. The vector preferably comprises a selectable marker to allow for selection of transformants, two recombination sites to allow recombination into another vector for the “second library of constructs”, e.g., a destination vector, and an origin of replication for propagation in a host organism, e.g.,  E. coli, Saccharomyces cerevisiae , or  Bacillus subtilis . In the case where the vector comprises two recombination sites, upon ligation of the polynucleotide of interest with the vector, the first recombination site flanks the 5′ end of the polynucleotide and the second recombination site flanks the 3′ end of the polynucleotide. Alternatively, the polynucleotide of interest can be modified to comprise a first recombination site flanking the 5′ end of the polynucleotide and a second recombination site flanking the 3′ end of the polynucleotide to facilitate site-specific recombination of the polynucleotide with a vector for the “second library of constructs”. For example, two att sites flanking the polynucleotide of interest may be incorporated for recombinase-mediated recombination. In a preferred aspect, the flanking sites consist of at least 3 nucleotides, preferably at least 19 nucleotides, more preferably at least 40 nucleotides, and most preferably at least 60 nucleotides.  
      In a preferred aspect, the PENTR™ Directional TOPO™ Cloning Kits available from Invitrogen, Carlsbad, Calif., are used in the methods of the present invention. Examples of vectors that may be employed in the present invention include, but are not limited to, pENTR™/D-TOPO, pENTR™/SD/D-TOPO, pENTR™/TEV/D-TOPO, pENTR™1A, pENTR™2B, PENTR™3C, pENTR™4, and pENTR™11. These vectors are known commercially as entry vectors.  
      The vector comprising the polynucleotide of interest is then transformed into a suitable host cell. Any host cell may be used in the methods of the present invention such as those host cells described herein for expression of a mutant polynucleotide. A preferable host cell is, but is not limited to,  E. coli, Saccharomyces cerevisiae , or  Bacillus subtilis . Transformants containing the vector with an insert in the correct orientation are then selected, and plasmid DNA isolated and analyzed by restriction analysis, PCR, and/or sequencing for the presence and correct orientation of the insert. Selecting the vector with an insert in the correct orientation enables directional subcloning from the vector into another vector, e.g., a destination vector.  
      The vector comprising the polynucleotide of interest is then subjected to insertional mutagenesis in the presence of an artificial transposon and a transposase to insert the artificial transposon at one or more random positions within the polynucleotide. The artificial transposon preferably comprises 5′ and 3′ conserved tandem inverted repeats which act as recognition sites for a transposase; a selectable marker gene located within the transposon sequence; and at least two restriction endonuclease recognition sites for transposon and selectable marker removal, and for introduction of one or more substitutions, deletions, or insertions, and self-ligation. Transposase recognition sequences are typically conserved tandom repeats that vary in size depending on the transposition system. For example, the TN7 transposon has two terminal 8-nucleotide inverted repeats.  
      The randomness of insertion of the transposable element into the polynucleotide of interest can be assessed by preparing DNA, e.g., cosmid DNA, and performing DNA sequencing directed from primers at either ends of the transposon.  
      The transposase can exist in two different forms. The transposase for Tn5 and Ty1 are made up of a single protein, as are most transposases, and is responsible for target site selection as well as the chemical reactions. In contrast, the Tn7 transposase is made up of several proteins. One set of Tn7 proteins is responsible for selecting the target sites and the other set of Tn7 proteins is needed to carry out the chemical steps of the reaction. A variety of transposases are known in the literature. For a discussion of transposase use and function, see Haren et al., 1999 , Annu. Rev. Microbiol.  53, 245-281.  
      In a preferred aspect, subcloning and expression of a transposase gene are performed from transposons such as Tn5, Tn7 or Mu in a suitable host cell.  
      Any transposon may be used in the methods of the present invention by modifying the transposon to comprise the above components.  
      Examples of transposons that may be so modified include, but are not limited to, three distinct types: (1) Retrotransposons (Class I) that first transcribe the DNA into RNA and then use reverse transcriptase to make a DNA copy of the RNA to insert in a new location.; (2) Class II transposons consisting only of DNA that moves directly from place to place; and (3) Class III transposons; also known as Miniature Inverted-Repeats Transposable Elements or MITEs.  
      A transposable element can be obtained from a suitable source using restriction enzymes and the components described above can be inserted into the transposable element so long as the insertion does not disrupt the inverted repeat sequences that are the binding site for the appropriate transposon. Transposons suitable in the present invention include, but are not limited to, those based upon the yeast Ty1 element, those based upon the bacterial transposon Tn7, the EZ::TN, those based on the bacteriophage Mu, those based on the bacterial transposon Tn552, and the mariner transposable element Himar1 (Lampe et al., 1998 , Genetics  149: 179-187), AT-2 (Perkin Elmer; Devine et al., 1997 , Genome Res.  7: 551-563), GPS-1 (New England Biolabs), and GPS-2 (New England Biolabs). A number of transposons and methods of identifying and isolating transposons are reviewed by Dyson, 1999 , Methods Microbiol.  29: 133-167, incorporated herein by reference. Although these specific transposon systems have been developed for use in in vitro systems, it is contemplated that many of the transposon systems, currently only available for in vivo transposition, may be modified and developed for in vitro work. With appropriate development and characterization, these in vivo transposon systems will also be suitable for use in the methods of the present invention.  
      Although any commercially available in vitro transposition system can be used as a mutagenizing tool, the Entranceposon M1-Cam® (Finnzymes Oy, Espoo, Finland) and the Mutation Generation System™ (MGS™, Finnzymes Oy, Espoo, Finland) are preferred to generate transposon insertions in the polynucleotide of interest. The Entranceposon M1-Cam® utilizes the bacteriophage Mu transposase to insert an artificial transposon at random positions within a target DNA population (Mizuuchi, 1992 , Annual Review of Biochemistry  61: 1011-1051; Haapa et al., 1999 , Nucleic Acids Research  27: 2727-2784). The artificial 1.254 kb transposon used in this system contains the following components: 44 bp 5′ and 3′ conserved tandem inverted repeats which act as recognition sites for the Mu transposase, Not I sites located within the inverted repeats that are used for transposon removal and self-ligation, and internal to these repeats is the coding sequence for a chloramphenicol selection marker.  
      Other kits for in vitro transposition that are commercially available include, for example, The Primer Island Transposition Kit, available from Perkin Elmer Applied Biosystems, Branchburg, N.J., based upon the yeast Ty1 element (including the AT2 transposon); The Genome Priming System, available from New England Biolabs, Beverly, Mass., based upon the bacterial transposon Tn7; and the EZ::TN Transposon Insertion Systems, available from Epicentre Technologies, Madison, Wis., based upon the Tn5 bacterial transposable element.  
      In the methods of the present invention, the first selectable marker may be any marker that is suitable for use in the host cell of choice. A selectable marker is a gene the product of which provides for biocide or viral resistance, resistance to heavy metals, prototrophy to auxotrophs, and the like to permit easy selection of transformed, transfected, transduced, or the like cells.  
      Examples of bacterial selectable markers are the dal genes from  Bacillus subtilis  or  Bacillus licheniformis , or markers which confer antibiotic resistance such as ampicillin, kanamycin, chloramphenicol, or tetracycline resistance. Suitable markers for yeast host cells are ADE2, HIS3, LEU2, LYS2, MET3, TRP1, and URA3. Selectable markers for use in a filamentous fungal host cell include, but are not limited to, amdS (acetamidase), argB (ornithine carbamoyltransferase), bar (phosphinothricin acetyltransferase), hph (hygromycin phosphotransferase), niaD (nitrate reductase), pyrG (orotidine-5′-phosphate decarboxylase), sC (sulfate adenyltransferase), and trpC (anthranilate synthase), as well as equivalents thereof. Preferred for use in an  Aspergillus  cell are the amdS and pyrG genes of  Aspergillus nidulans  or  Aspergillus oryzae  and the bar gene of  Streptomyces hygroscopicus.    
      Second Library. A second library of constructs is isolated by introducing the first library of constructs into a vector comprising a third selectable marker and a first recombination site and a second recombination site to facilitate site-specific recombination of the first recombination site flanking the 5′ end of the polynucleotide and the second recombination site flanking the 3′ end of the polynucleotide in the first library of constructs with the first recombination site and the second recombination site of the vector and by selecting the second library of constructs using the second and third selectable markers in a suitable host cell.  
      The recombination reaction is performed in the presence of a recombinase and a vector for the “second library of constructs”, e.g., a destination vector, to transfer the polynucleotides from the first library of constructs into the vector to generate a second library of constructs or expression clones. Site-specific recombination of the first recombination site flanking the 5′ end of the polynucleotide and the second recombination site flanking the 3′ end of the polynucleotide in the first library of constructs occurs with the first recombination site and the second recombination site of the vector. The second library of constructs is then selected using the second and third selectable markers.  
      Any recombinase may be used in the methods of the present invention. In a preferred aspect, LR Clonase™ (Invitrogen, Carlsbad, Calif.) is used as the recombinase in the present invention. LR Clonase™ is an enzyme mix containing bacteriophage lambda recombination proteins Integrase amd Excisionase and the  E. coli -encoded protein Integration Host Factor.  
      Any vector for the “second library of constructs” can be used in the methods of the present invention, such as a destination vector. A large selection of Gateway™ destination vectors are available from Invitrogen, Carlsbad, Calif. The vector for the “second library of constructs” preferably comprises a promoter for expression in the host of choice, e.g., yeast GAL1 promoter for galactose-inducible expression in  Saccharomyces cerevisiae ; two recombination sites preferably downstream of the promoter for recombinational cloning of the polynucleotide of interest from the vector for the “first library of constructs”; a selectable marker, e.g., chloramphenicol resistance gene, located between the two recombination sites; and an origin of replication for plasmid maintenance in the host. The two recombination sites in the vector for the “second library of constructs” will be the same as or highly homologous to the two recombination sites in the vector for the “first library of constructs”. The vector may further comprise one or more of the following components: a negative selection marker, e.g., ccdB gene, located between the two recombination sites; a polyadenylation sequence for proper termination and processing of the recombinant transcript; an origin for episomal maintenance and high copy replication, e.g., a 2μ origin; an auxotrophic marker for selection in yeast, e.g., URA3 auxotrophic marker; an origin for high copy replication and maintenance of the plasmid in  E. coli , e.g., pUC origin; and a gene for selection in  E. coli , e.g., ampicillin resistance gene.  
      Any promoter capable of driving expression of the polynucleotide is suitable for the present invention. Preferred promoters include, but not limited to, CYC1, HIS3, GAL1, GAL10, ADH1, PGK, PHO5, GAPDH, ADC1, TRP1, URA3, LEU2, ENO, TPI (useful for expression in  Saccharomyces ); AOX1 (useful for expression in  Pichia ); and lac, ara, tet, trp, IP L , IP R , T7, tac, and trc (useful for expression in  Escherichia coli ) as well as the amy, apr, and npr promoters and various phage promoters useful for expression in  Bacillus.    
      Examples of destination vectors particularly useful in the present invention include, but are not limited to, pBAD-DEST49, pET-DEST42, pDEST14, PDEST™15, pDEST™17, pDEST™24, and pYES2-DEST52.  
      The recombination reaction between the two recombination sites on the vector for the “first library of constructs” and the two recombination sites on the vector for the “second library of constructs” preferably replaces the selectable marker gene and the negative selectable marker gene, if present, with the polynucleotide of interest comprising recombination sites in the expression clone.  
      Following the recombination reaction, the reaction mixture is preferably transformed into a suitable host cell to select for expression clones. Any host cell may be used such as those host cells described herein for expression of a mutant polynucleotide.  
      In a preferred aspect, competent  E. coli  are used to select for expression clones. Any recA, endA  E. coli  strain including  E. coli  TOP10, DH5α, DH10B, or an equivalent strain, may be used for transformation of the reaction mixture. In the case where the vector for the “second library of constructs” contains a ccdB gene for negative selection,  E. coli  strains that contain the F′ episome cannot be used.  
      In the methods of the present invention, the second and third selectable markers may be any marker that is suitable for use in the host cell of choice as long as they are different from each other and the first selectable marker. Selection with the second and third selectable markers eliminates propagation of the first library of constructs in the second library of constructs.  
      Transposon mutagenesis, as described herein, can be used to create polynucleotide insertions, deletions, or substitutions by selectively removing some or all or more than the inserted transposon. Using natural or artificial transposons containing restriction endonucleases sites, the inserted transposon and/or target polynucleotide can be selectively cleaved to remove some or all or more than the inserted transposon, and then religated to create the desired insertion, deletion, or substitution. The choice of restriction enzyme or enzymes to be used will depend on whether a substitution, a deletion, or an insertion is being introduced. Roberts et al., 2003 , Nucleic Acids Research  31: 418-420 describes various types of restriction endonucleases. Restriction endonucleases can be obtained from numerous commercial suppliers.  
      By applying transposon technology combined with both Type II restriction endonucleases (restriction endonucleases cutting inside their recognition sequence, hereafter referred to as “inside cutters” as defined herein), it is possible to produce a targeted polynucleotide with one or more nucleotide insertions. Insertions occur wherein the transposon comprises two or more Type II restriction endonuclease recognition sites. For insertions, in theory, any number of nucleotides can be inserted depending on the location of restriction endonuclease cleavage sites within the transposon and subsequent ligation of the remaining transposon.  
      By applying transposon technology combined with Type IIS or Type IIG restriction endonucleases or any other restriction endonuclease that cleaves a polynucleotide outside their recognition sequence (hereafter referred to as “outside cutters” as defined herein), it is possible to produce targeted polynucleotide libraries with one or more nucleotide deletions. Deletions can be generated when two outside cutter recognition sites are positioned within the inserted transposon such that the outside cutters cleave the target polynucleotide. Religation of the resulting cleavage of the resulting polynucleotide containing the target polynucleotide then results in a mutagenized target polynucleotide deleted in one or more nucleotides.  
      By applying transposon technology combined with outside cutters, it is also possible to produce targeted polynucleotide libraries with one or more substitutions. For substitutions, one or more nucleotides may be substituted with alternate nucleotides to provide a substitution targeted polynucleotide library.  
      Substitutions can occur where the transposon comprises two or more outside cutter recognition sites; and more preferably at least one of the one or more outside cutter recognition sites are located so that cleavage with at least one corresponding outside cutter restriction endonuclease results in at least one cut in the targeted polynucleotide located outside of the transposon. By addition and ligation of a linker consisting of a number of nucleotides, subject to the number of nucleotides in the targeted polynucleotide that are removed by cleavage of the outside cutters, one or more substitutions result.  
      Substitutions can also occur where the use of one or more outside cutter restriction endonucleases results in cleavage of the targeted polynucleotide sequence leaving a set number of nucleotides between the cleavage site and one of the two transposon insertion junctions followed by the use of one or more restriction endonucleases which results in the cleavage of the entire transposon minus the number of nucleotides that are between the cleavage site of the outside cutter restriction endonuclease and one of the two transposon junction sites. Religation of the resulting cleavage of the resulting polynucleotide containing the target polynucleotide then results in a mutagenized target polynucleotide substituted in one or more nucleotides.  
      Insertion Library. In the methods of the present invention, an insertion library containing at least one substitution, deletion, or insertion of at least one nucleotide in each polynucleotide of the second library of constructs is isolated by removing all, essentially all, or a portion of the inserted artificial transposon from the second library of constructs through restriction endonuclease digestion of the at least two restriction endonuclease recognition sites leaving at least one substitution, deletion, or insertion of at least one nucleotide in the polynucleotide; self-ligating the restriction endonuclease digested fragments; and selecting the insertion library using the third selection marker in a suitable host cell.  
      The choice of restriction enzyme or enzymes to be used in creating the insertion library will depend on whether a substitution, a deletion, or an insertion is being introduced, as described earlier.  
      For example, in the Entranceposon M1-Cam® System (Finnzymes Oy, Espoo, Finland), the transposon, after insertion, can be removed using the restriction enzyme Not I followed by self-ligation of the backbone which results in a 15 bp in-frame insertion. Ten of 15 bps inserted originate from the inverted repeat sequence that flanks the transposon. The other 5 bp are a result of duplication of the target site that occurs upon integration. The five amino acid insert can be translated into three different peptide combinations based on the insertion frame. In one frame three of the five amino acids are alanines, which is a desired outcome for less deleterious changes to the overall structure of a protein.  
      In the methods of the present invention, the third selectable marker may be any marker that is suitable for use in the host cell of choice as long as it is different from the first and second selectable markers.  
      Any host cell may be used in the methods of the present invention such as those host cells described herein for expression of a mutant polynucleotide. A preferable host cell is, but is not limited to,  E. coli, Saccharomyces cerevisiae , or  Bacillus subtilis.    
      In a second aspect, the present invention relates to methods of producing at least one polynucleotide encoding at least one variant of a parent polypeptide, the method comprising the steps of: 
          (a) providing a nucleic acid construct comprising a polynucleotide encoding the parent polypeptide, into which polynucleotide has been inserted a heterologous polynucleotide fragment, wherein said fragment comprises at least two restriction endonuclease recognition sites;     (b) restricting the nucleic acid construct with at least two corresponding restriction endonucleases, if necessary in separate individual steps of restricting, PCR-polishing, and ligating, wherein all or essentially all of the inserted heterologous fragment is excised from the construct and at least one nucleotide triplet is deleted, inserted, or substituted in the encoding polynucleotide in the process, whereby at least one polynucleotide encoding at least one variant of the parent polypeptide is produced.        

      Codon triplets and diversity. For a medium sized protein of typically 400 amino acids, a full library covering a single amino acid substitution in one position would be relatively small: 400×20=8,000 polypeptides, which corresponds to 25,600 polynucleotide coding sequences (using 64 codon triplets). To cover the theoretical diversity in all three reading frames would therefore require 76,800 DNA combinations.  
      When it is considered that some transposons are inserted into their targets randomly and in either orientation, irrespective of the reading frame, and that a random or partially random codon triplet ‘NNN’ introduced by the transposon can therefore end up in both orientations and in all reading frames, then the theoretical coding diversity of the ‘NNN’ triplet can be limited to only 22 codons (excluding stop-codons) in the transposon, rather than 64. For example, if the codon for Trp ‘TGG’ is positioned to be substituted in one orientation of transposon, the other orientation of transposon would result in the codon ‘CCA’ (Pro) in the opposite orientation.  
      Consequently, all twenty amino acid substitutions can in this way be coded for by only 22 different codons in a transposon, as shown in Table 1 below. For a medium sized protein of 400 amino acids the theoretical diversity for all three reading frames would therefore be only 26,400 DNA combinations.  
               TABLE 1                          The 22 codons represent all 20 amino acids without stop codons and       with only two amino acids (Phe, Val) represented twice. The column       ‘Codon-1’ shows the codons (one direction) for amino acids in       column ‘AA-1’ and the codons in column ‘Codon-2’ are the       complement triplets of the codons (opposite direction) in ‘Codon-1’ and       they code for the amino acids in ‘AA-2’.                                     AA-1   Codon 1   Codon 2   AA-2                       Trp   TGG   CCA   Pro           Met   ATG   CAT   His           Asp   GAT   ATC   Ile           Asn   AAC   GTT   Val           Lys   AAA   TTT   Phe           Glu   GAA   TTC   Phe           Tyr   TAC   GTA   Val           Gln   CAA   TTG   Leu           Cys   TGT   ACA   Thr           Ala   GCC   GGC   Gly           Ser   TCG   CGA   Arg                      
 
      The method of the second aspect comprises several steps, the first of which is the insertion of a transposon into a gene of interest, which gene is preferably located on a plasmid, as described earlier, and which may have been modified to remove any unwanted restriction enzyme sites and/or introns. Gene-fragments with an inserted transposon are then isolated and cloned into a vector, as described earlier. The inserted transposon, which is flanked by restriction enzyme sites, is then replaced in the gene of interest by use of the restriction enzyme(s), e.g. Not I as illustrated in  FIG. 1 .  
      A DNA fragment is designed and manufactured comprising a random or partially random triplet codon ‘NNN’ flanked by “outside cutting” restriction enzyme sites that are flanked in turn by restriction enzyme sites compatible to those flanking the transposon. Alternatively, the transposon may be modified to comprise the outside cutter sites prior to its insertion by transposition into the gene of interest.  
      For the production of a library of polynucleotides encoding polypeptides having one or more amino acid insertions or substitutions, the use of random or partially random codon triplets is advantageous, often denoted ‘NNN’. They may consist of a sharply defined ratio of nucleotides in each position. If the composition in one position is 25% A, 25% G, 25% C, and 25% T, the position is said to be random, i.e., the likelihood is the same for any nucleotide to be present there. However, the ratios may also be adjusted to prefer one or more nucleotides in a given position, in which case it is merely partially random.  
      Accordingly, in a preferred embodiment, the heterologous polynucleotide fragment or the transposon comprises at least one random or partially random codon triplet ‘NNN’.  
      In another preferred embodiment, the at least two restriction endonuclease recognition sites comprise one or more outside cutter restriction endonuclease recognition site, and preferably restriction with the one or more corresponding outside cutter endonuclease results in one or more cut in the polynucleotide outside of the inserted heterologous polynucleotide fragment.  
      Another preferred embodiment relates to the method of the second aspect, wherein the at least two restriction endonuclease recognition sites comprise two or more different outside cutter restriction endonuclease recognition sites.  
      The DNA fragment and a plasmid comprising the gene with the inserted transposon are digested with the compatible restriction enzymes and the DNA fragment is cloned into the gene to replace the transposon.  
      The outside-cutting sites flanking the inserted DNA-fragment are then restricted with the appropriate outside cutter, if necessary the restricted DNA ends are blunt-ended or filled-in, e.g., by PCR polishing, to enable the subsequent ligation (see  FIG. 7 ).  
      Finally the inserted DNA-fragment is excised from the construct by another outside cutter and the construct is ligated, if necessary after the fragments have been blunt-ended or filled-in, so that the three random or partially random base pairs substitute three base pairs of the coding sequence and nothing else of the inserted DNA remains in the construct.  
      In the resulting polynucleotide only the random or partially random codon triplet ‘NNN’ remains of the DNA inserted into the gene. This triplet has been brought into position in the coding sequence of the gene of interest and in the process it has replaced three nucleotides of the coding sequence (see  FIG. 7 ).  
      Naturally, more than one codon triplet may be substituted at one time, and by designing the location of the outside cutter recognition sites properly one or more codon triplet may also be inserted and/or deleted. When deletions are intended, all the inserted heterologous sequence will be excised in the process. To achieve insertions or substitutions essentially all of the inserted heterologous sequence will be excised in the process, but of course the respective heterologous inserting and/or substituting coding triplets will necessarily have to be left behind.  
      In a preferred embodiment, the heterologous polynucleotide fragment comprises a transposon.  
      In another preferred embodiment, the construct is a DNA plasmid.  
      In another preferred embodiment, the heterologous polynucleotide fragment or the transposon comprises a selection marker, preferably an antibiotic resistance marker.  
      In another preferred embodiment, the heterologous polynucleotide fragment or the transposon comprises a polynucleotide having the sequence shown in SEQ ID NO: 10.  
      A third aspect of the present invention relates to a polynucleotide construct comprising a transposon, said transposon comprising one or more outside cutter restriction endonuclease recognition sites.  
      The nucleic construct of the third aspect may represent a means for carrying out the method of the second aspect. However, it may also represent an intermediary result after step (a) in the method of the second aspect.  
      A preferred embodiment of the third aspect is that the transposon comprises two or more outside cutter restriction endonuclease recognition sites; preferably the transposon comprises two or more different outside cutter restriction endonuclease recognition sites; and more preferably at least one of the one or more outside cutter restriction endonuclease recognition site is located so that restriction with at least one corresponding outside cutter restriction endonuclease results in at least one cut in the polynucleotide construct outside of the transposon.  
      In a fourth aspect the present invention relates to a cell comprising in its genome an integrated heterologous polynucleotide fragment, said fragment comprising one or more outside cutter restriction endonuclease recognition sites.  
      The cell of the fourth aspect may also represent a means for carrying out the method of the first aspect, but also an intermediary result after step (a) in the method of the second aspect.  
      In a preferred embodiment of the fourth aspect, the heterologous polynucleotide fragment comprises a transposon, wherein the one or more outside cutter restriction endonuclease recognition site is comprised in the transposon; preferably the heterologous polynucleotide fragment comprises two or more outside cutter restriction endonuclease recognition sites; and more preferably the heterologous polynucleotide fragment comprises two or more different outside cutter restriction endonuclease recognition sites.  
      In another preferred embodiment of the fourth aspect, at least one of the one or more outside cutter restriction endonuclease recognition site is located so that restriction with at least one corresponding outside cutter restriction endonuclease results in at least one cut in the genome of the cell outside of the integrated heterologous polynucleotide fragment.  
      Polynucleotides  
      The polynucleotide of interest can be any polynucleotide and can be obtained from any prokaryotic, eukaryotic, or other source. For purposes of the present invention, the term “obtained from” as used herein in connection with a given source shall mean that the polynucleotide is native to the source or is from a source into which the polynucleotide had been inserted. In a preferred aspect, the polynucleotide of interest encodes a polypeptide that is secreted extracellularly.  
      Techniques used to isolate or clone a polynucleotide of interest are known in the art and include isolation from genomic DNA, preparation from cDNA, or a combination thereof. The cloning of the polynucleotide from such genomic DNA can be effected, e.g., by using the well known polymerase chain reaction (PCR) or antibody screening of expression libraries to detect cloned DNA fragments with shared structural features. See, e.g., Innis et al., 1990 , PCR: A Guide to Methods and Application , Academic Press, New York. Other nucleic acid amplification procedures such as ligase chain reaction (LCR), ligated activated transcription (LAT), and nucleotide sequence-based amplification (NASBA) may be used. Standard recombinant DNA and molecular cloning techniques used herein are well known in the art and are described by Sambrook, J., Fritsch, E. F. and Maniatis, T., Molecular Cloning: A Laboratory Manual, Second Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (1989); and by Silhavy, T. J., Bennan, M. L. and Enquist, L. W., Experiments with Gene Fusions, Cold Spring Harbor Laboratory Cold Press Spring Harbor, N.Y. (1984); and by Ausubel, F. M. et al., Current Protocols in Molecular Biology, published by Greene Publishing Assoc. and Wiley-Interscience (1987). The polynucleotide may be of genomic, cDNA, RNA, semisynthetic, synthetic origin, or any combinations thereof.  
      The polynucleotide of interest may encode a polypeptide such as an antibody, hormone, enzyme, receptor, reporter, or selectable marker. The polypeptide is preferably secreted extracellularly.  
      In a preferred aspect, the polypeptide is an oxidoreductase, transferase, hydrolase, lyase, isomerase, or ligase. In a more preferred aspect, the polypeptide is an aminopeptidase, amylase, beta-glucosidase, carbohydrase, carboxypeptidase, catalase, cellobiohydrolase, cellulase, chitinase, cutinase, cyclodextrin glycosyltransferase, deoxyribonuclease, endoglucanase, esterase, alpha-galactosidase, beta-galactosidase, glucoamylase, alpha-glucosidase, beta-glucosidase, invertase, laccase, lactonohydrolase, lipase, lysozyme, mannosidase, mutanase, oxidase, pectinolytic enzyme, peroxidase, phospholipase, phytase, polyphenoloxidase, proteolytic enzyme, ribonuclease, transglutaminase, or xylanase.  
      A polypeptide can also include fused polypeptides or cleavable fusion polypeptides in which another polypeptide is fused at the N-terminus or the C-terminus of a polypeptide or fragment thereof. A fused polypeptide is produced by fusing another nucleotide sequence (or a portion thereof) encoding another polypeptide to a nucleotide sequence (or a portion thereof) encoding a polypeptide. Techniques for producing fusion polypeptides are known in the art, and include ligating the coding sequences encoding the polypeptides so that they are in frame and that expression of the fused polypeptide is under control of the same promoter(s) and terminator.  
      The polynucleotide of interest can also be a control sequence such as a leader, polyadenylation sequence, propeptide sequence, promoter, signal peptide sequence, or transcription terminator.  
      The polynucleotide of interest can also be an origin of replication.  
      The polynucleotide of interest may be bacterial in origin. For example, the polynucleotide may be obtained from a gram positive bacterium such as a  Bacillus  or  Streptomyces , or a gram negative bacterium.  
      In a preferred aspect, the polynucleotide is obtained from  Bacillus alkalophilus, Bacillus amyloliquefaciens, Bacillus brevis, Bacillus circulans, Bacillus coagulans, Bacillus lautus, Bacillus lentus, Bacillus licheniformis, Bacillus megaterium, Bacillus stearothermophilus, Bacillus subtilis, Bacillus thuringiensis, Streptomyces lividans , or  Streptomyces murinus . In another preferred aspect, the polynucleotide is obtained from  E. coli  or  Pseudomonas  sp.  
      The polynucleotide of interest may also be fungal in origin, and preferably from a yeast such as  Candida, Kluyveromyces, Pichia, Saccharomyces, Schizosaccharomyces , or  Yarrowia ; or preferably from a filamentous fungus such as  Acremonium, Aspergillus, Aureobasidium, Cryptococcus, Filibasidium, Fusarium, Humicola, Magnaporthe, Mucor, Myceliophthora, Neocallimastix, Neurospora, Paecilomyces, Penicillium, Piromyces, Schizophyllum, Talaromyces, Thermoascus, Thielavia, Tolypocladium , or  Trichoderma.    
      In a preferred aspect, the polynucleotide is obtained from  Saccharomyces carlsbergensis, Saccharomyces cerevisiae, Saccharomyces diastaticus, Saccharomyces douglasii, Saccharomyces kluyveri, Saccharomyces norbensis , or  Saccharomyces oviformis.    
      In another preferred aspect, the polynucleotide is obtained from  Aspergillus aculeatus, Aspergillus awamori, Aspergillus fumigatus, Aspergillus foetidus, Aspergillus japonicus, Aspergillus nidulans, Aspergillus niger, Aspergillus oryzae, Fusarium bactridioides, Fusarium cerealis, Fusarium crookwellense, Fusarium culmorum, Fusarium graminearum, Fusarium graminum, Fusarium heterosporum, Fusarium negundi, Fusarium oxysporum, Fusarium reticulatum, Fusarium roseum, Fusarium sambucinum, Fusarium sarcochroum, Fusarium sporotrichioides, Fusarium sulphureum, Fusarium torulosum, Fusarium trichothecioides, Fusarium venenatum, Humicola insolens, Humicola lanuginosa, Mucor miehei, Myceliophthora thermophila, Neurospora crassa, Penicillium purpurogenum, Trichoderma harzianum, Trichoderma koningii, Trichoderma longibrachiatum, Trichoderma reesei , or  Trichoderma viride.    
      It will be understood that for the aforementioned species the invention encompasses both the perfect and imperfect states, and other taxonomic equivalents, e.g., anamorphs, regardless of the species name by which they are known. Those skilled in the art will readily recognize the identity of appropriate equivalents.  
      Strains of these species are readily accessible to the public in a number of culture collections, such as the American Type Culture Collection (ATCC), Deutsche Sammlung von Mikroorganismen und Zelikulturen GmbH (DSM), Centraalbureau Voor Schimmelcultures (CBS), and Agricultural Research Service Patent Culture Collection, Northern Regional Research Center (NRRL).  
      A polynucleotide of interest may be identified and obtained from other sources including microorganisms isolated from nature (e.g., soil, composts, water, etc.) using the above-mentioned probes. Techniques for isolating microorganisms from natural habitats are well known in the art. The polynucleotide may then be obtained by similarly screening a genomic or cDNA library of another microorganism. Once a polynucleotide sequence encoding a polypeptide has been detected with the probe(s), the polynucleotide can be isolated or cloned by utilizing techniques which are well known to those of ordinary skill in the art (see, e.g., Sambrook et al., 1989, supra).  
      Isolation of a Mutant of the Polynucleotide  
      Techniques used to isolate or clone a mutant of a polynucleotide of interest from the insertion library are known in the art and include isolation from genomic DNA, preparation from cDNA, or a combination thereof. The cloning of the polynucleotide from such genomic DNA can be effected, e.g., by using the well known polymerase chain reaction (PCR) or antibody screening of expression libraries to detect cloned DNA fragments with shared structural features. See, e.g., Innis et al., 1990, supra. Other nucleic acid amplification procedures such as ligase chain reaction (LCR), ligated activated transcription (LAT) and nucleotide sequence-based amplification (NASBA) may be used.  
      Conventions for Designation of Variants  
      In the present invention, specific numbering of amino acid residue positions is employed in the protein variants. For example, by aligning the amino acid sequences of known proteins having the same biological function, it is possible to designate an amino acid position number to any amino acid residue in any specific protein.  
      Multiple alignments of protein sequences may be made, for example, using “Clustal W” (Thompson, J. D., Higgins, D. G. and Gibson, T. J., 1994, CLUSTAL W: Improving the sensitivity of progressive multiple sequence alignment through sequence weighting, positions-specific gap penalties and weight matrix choice,  Nucleic Acids Research  22: 4673-4680). Multiple alignments of DNA sequences may be done using the protein alignment as a template, replacing the amino acids with the corresponding codon from the DNA sequence.  
      Pairwise sequence comparison algorithms in common use are adequate to detect similarities between protein sequences that have not diverged beyond the point of approximately 20-30% sequence identity (Doolittle, 1992 , Protein Sci.  1: 191-200; Brenner et al., 1998 , Proc. Natl. Acad. Sci. USA  95, 6073-6078). However, truly homologous proteins with the same fold and similar biological function have often diverged to the point where traditional sequence-based comparisons fail to detect their relationship (Lindahl and Elofsson, 2000 , J. Mol. Biol.  295: 613-615). Greater sensitivity in sequence-based searching can be attained using search programs that utilize probabilistic representations of protein families (profiles) to search databases. For example, the PSI-BLAST program generates profiles through an iterative database search process and is capable of detecting remote homologs (Atschul et al., 1997 , Nucleic Acids Res.  25: 3389-3402). Even greater sensitivity can be achieved if the family or superfamily for the protein of interest has one or more representatives in the protein structure databases. Programs such as GenTHREADER (Jones 1999 , J. Mol. Biol.  287: 797-815; McGuffin and Jones, 2003 , Bioinformatics  19: 874-881) utilize information from a variety of sources (PSI-BLAST, secondary structure prediction, structural alignment profiles, and solvation potentials) as input to a neural network that predicts the structural fold for a query sequence. Similarly, the method of Gough et al., 2000 , J. Mol. Biol.  313: 903-919, can be used to align a sequence of unknown structure with the superfamily models present in the SCOP database. These alignments can in turn be used to generate homology models for the protein of interest, and such models can be assessed for accuracy using a variety of tools developed for that purpose.  
      For proteins of known structure, several tools and resources are available for retrieving and generating structural alignments. For example the SCOP superfamilies of proteins have been structurally aligned, and those alignments are accessible and downloadable. These alignments can be used to predict the structurally and functionally corresponding amino acid residues in proteins within the same structural superfamily. This information, along with information derived from homology modeling and profile searches, can be used to predict which residues to mutate when moving mutations of interest from one protein to a close or remote homolog.  
      In describing the protein variants of the present invention, the nomenclature described below is adapted for ease of reference. In all cases, the accepted IUPAC single letter or triple letter amino acid abbreviation is employed.  
      Substitutions. For an amino acid substitution, the following nomenclature is used: Original amino acid, position, substituted amino acid. Accordingly, the substitution of threonine with alanine at position 226 is designated as “Thr226Ala” or “T226A”. Multiple mutations are separated by addition marks (“+”), e.g., “Gly205Arg+Ser411 Phe” or “G205R+S411F”, representing mutations at positions 205 and 411 substituting glycine (G) with arginine (R), and serine (S) with phenylalanine (F), respectively.  
      Deletions. For an amino acid deletion, the following nomenclature is used: Original amino acid, position*. Accordingly, the deletion of glycine at position 195 is designated as “Gly195*” or “G195*”. Multiple deletions are separated by addition marks (“+”), e.g., “Gly195*+Ser411*” or “G195*+S411*”.  
      Insertions. For an amino acid insertion, the following nomenclature is used: Original amino acid, position, original amino acid, new inserted amino acid. Accordingly the insertion of lysine after glycine at position 195 is designated “Gly195GlyLys” or “G195GK”.  
      Multiple modifications. Variants comprising multiple modifications are separated by addition marks (“+”), e.g., “Arg170Tyr+Gly195Glu” or “R170Y+G195E” representing modifications at positions 170 and 195 substituting tyrosine and glutamic acid for arginine and glycine, respectively.  
      The artificial variants may comprise a conservative substitution, deletion, and/or insertion of one or more amino acids that, for example, do not significantly affect the folding and/or activity of the protein; small deletions, typically of one to about 30 amino acids; or small amino- or carboxyl-terminal extensions, such as an amino-terminal methionine residue.  
      Examples of conservative substitutions are within the group of basic amino acids (arginine, lysine and histidine), acidic amino acids (glutamic acid and aspartic acid), polar amino acids (glutamine and asparagine), hydrophobic amino acids (leucine, isoleucine and valine), aromatic amino acids (phenylalanine, tryptophan and tyrosine), and small amino acids (glycine, alanine, serine, threonine and methionine). Amino acid substitutions which do not generally alter specific activity are known in the art and are described, for example, by H. Neurath and R. L. Hill, 1979, In, The Proteins, Academic Press, New York. The most commonly occurring exchanges are Ala/Ser, Val/Ile, Asp/Glu, Thr/Ser, Ala/Gly, Ala/Thr, Ser/Asn, Ala/Val, Ser/Gly, Tyr/Phe, Ala/Pro, Lys/Arg, Asp/Asn, Leu/Ile, Leu/Val, Ala/Glu, and Asp/Gly.  
      In addition to the 20 standard amino acids, non-standard amino acids (such as 4-hydroxyproline, 6-N-methyl lysine, 2-aminoisobutyric acid, isovaline, and alpha-methyl serine) may be substituted for amino acid residues of a wild-type polypeptide. A limited number of non-conservative amino acids, amino acids that are not encoded by the genetic code, and unnatural amino acids may be substituted for amino acid residues. “Unnatural amino acids” have been modified after protein synthesis, and/or have a chemical structure in their side chain(s) different from that of the standard amino acids. Unnatural amino acids can be chemically synthesized, and preferably, are commercially available, and include pipecolic acid, thiazolidine carboxylic acid, dehydroproline, 3- and 4-methylproline, and 3,3-dimethylproline.  
      Alternatively, the amino acid changes are of such a nature that the physico-chemical properties of the polypeptides are altered. For example, amino acid changes may improve the thermal stability of the polypeptide, alter the substrate specificity, change the pH optimum, and the like. The artificial variants may comprise a substitution, deletion, and/or insertion of one or more essential amino acids in the parent polypeptide. Essential amino acids can be identified according to procedures known in the art, such as site-directed mutagenesis or alanine-scanning mutagenesis (Cunningham and Wells, 1989,  Science  244: 1081-1085). The active site of the enzyme or other biological interaction can also be determined by physical analysis of structure, as determined by such techniques as nuclear magnetic resonance, crystallography, electron diffraction, or photoaffinity labeling, in conjunction with mutation of putative contact site amino acids. See, for example, de Vos et al., 1992 , Science  255: 306-312; Smith et al., 1992 , J. Mol. Biol.  224: 899-904; Wlodaver et al., 1992 , FEBS Lett.  309:59-64. The identities of essential amino acids can also be inferred from analysis of identities with polypeptides which are related to a polypeptide according to the invention.  
      In a preferred embodiment, a mutant polynucleotide or a variant polypeptide has an improved property compared to the parent polynucleotide or the parent polypeptide, respectively. Such improved properties include, but are not limited to, altered control sequence function, altered temperature-dependent activity profile, thermostability, pH activity, pH stability, substrate specificity, product specificity, and chemical stability.  
      Nucleic Acid Constructs  
      The present invention also relates to nucleic acid constructs comprising an isolated mutant polynucleotide encoding an artificial variant of a parent polypeptide operably linked to one or more control sequences which direct the expression of the coding sequence in a suitable host cell under conditions compatible with the control sequences.  
      An isolated mutant polynucleotide encoding an artificial variant of the present invention may be manipulated in a variety of ways to provide for expression of the artificial variant. Manipulation of the polynucleotide&#39;s sequence prior to its insertion into a vector may be desirable or necessary depending on the expression vector. The techniques for modifying polynucleotide sequences utilizing recombinant DNA methods are well known in the art.  
      The control sequence may be an appropriate promoter sequence, a nucleotide sequence which is recognized by a host cell for expression of a mutant polynucleotide encoding an artificial variant of a polypeptide. The promoter sequence contains transcriptional control sequences which mediate the expression of the polypeptide. The promoter may be any nucleotide sequence which shows transcriptional activity in the host cell of choice including mutant, truncated, and hybrid promoters, and may be obtained from genes encoding extracellular or intracellular polypeptides either homologous or heterologous to the host cell.  
      Examples of suitable promoters for directing the transcription of the nucleic acid constructs of the present invention, especially in a bacterial host cell, are the promoters obtained from the  E. coli  lac operon,  Streptomyces coelicolor  agarase gene (dagA),  Bacillus subtilis  levansucrase gene (sacB),  Bacillus licheniformis  alpha-amylase gene (amyL),  Bacillus stearothermophilus  maltogenic amylase gene (amyM),  Bacillus amyloliquefaciens  alpha-amylase gene (amyQ),  Bacillus licheniformis  penicillinase gene (penP),  Bacillus subtilis  xylA and xylB genes, and prokaryotic beta-lactamase gene (Villa-Kamaroff et al., 1978 , Proceedings of the National Academy of Sciences USA  75: 3727-3731), as well as the tac promoter (DeBoer et al., 1983 , Proceedings of the National Academy of Sciences USA  80: 21-25). Further promoters are described in “Useful proteins from recombinant bacteria” in  Scientific American,  1980, 242: 74-94; and in Sambrook et al., 1989, supra.  
      Examples of suitable promoters for directing the transcription of the nucleic acid constructs of the present invention in a filamentous fungal host cell are promoters obtained from the genes for  Aspergillus oryzae  TAKA amylase,  Rhizomucor miehei  aspartic proteinase,  Aspergillus niger  neutral alpha-amylase,  Aspergillus niger  acid stable alpha-amylase,  Aspergillus niger  or  Aspergillus awamori  glucoamylase (glaA),  Rhizomucor miehei  lipase,  Aspergillus oryzae  alkaline protease,  Aspergillus oryzae  triose phosphate isomerase,  Aspergillus nidulans  acetamidase,  Fusarium venenatum  amyloglucosidase (WO 00/56900),  Fusarium venenatum  Daria (WO 00/56900),  Fusarium venenatum  Quinn (WO 00/56900),  Fusarium oxysporum  trypsin-like protease (WO 96/00787),  Trichoderma reesei  beta-glucosidase,  Trichoderma reesei  cellobiohydrolase I,  Trichoderma reesei  endoglucanase 1,  Trichoderma reesei  endoglucanase II,  Trichoderma reesei  endoglucanase III,  Trichoderma reesei  endoglucanase IV,  Trichoderma reesei  endoglucanase V,  Trichoderma reesei  xylanase 1,  Trichoderma reesei  xylanase II,  Trichoderma reesei  beta-xylosidase, as well as the NA2-tpi promoter (a hybrid of the promoters from the genes for  Aspergillus niger  neutral alpha-amylase and  Aspergillus oryzae  triose phosphate isomerase); and mutant, truncated, and hybrid promoters thereof.  
      In a yeast host, useful promoters are obtained from the genes for  Saccharomyces cerevisiae  enolase (ENO-1),  Saccharomyces cerevisiae  galactokinase (GAL1),  Saccharomyces cerevisiae  alcohol dehydrogenase/glyceraldehyde-3-phosphate dehydrogenase (ADH1, ADH2/GAP),  Saccharomyces cerevisiae  triose phosphate isomerase (TPI),  Saccharomyces cerevisiae  metallothionine (CUP1), and  Saccharomyces cerevisiae  3-phosphoglycerate kinase. Other useful promoters for yeast host cells are described by Romanos et al., 1992, Yeast 8: 423-488.  
      The control sequence may also be a suitable transcription terminator sequence, a sequence recognized by a host cell to terminate transcription. The terminator sequence is operably linked to the 3′ terminus of the nucleotide sequence encoding the artificial variant of a polypeptide. Any terminator which is functional in the host cell of choice may be used in the present invention.  
      Preferred terminators for filamentous fungal host cells are obtained from the genes for  Aspergillus oryzae  TAKA amylase,  Aspergillus niger  glucoamylase,  Aspergillus nidulans  anthranilate synthase,  Aspergillus niger  alpha-glucosidase, and  Fusarium oxysporum  trypsin-like protease.  
      Preferred terminators for yeast host cells are obtained from the genes for  Saccharomyces cerevisiae  enolase,  Saccharomyces cerevisiae  cytochrome C (CYC1), and  Saccharomyces cerevisiae  glyceraldehyde-3-phosphate dehydrogenase. Other useful terminators for yeast host cells are described by Romanos et al., 1992, supra.  
      The control sequence may also be a suitable leader sequence, a nontranslated region of an mRNA which is important for translation by the host cell. The leader sequence is operably linked to the 5′ terminus of the nucleotide sequence encoding the artificial variant of a polypeptide. Any leader sequence that is functional in the host cell of choice may be used in the present invention.  
      Preferred leaders for filamentous fungal host cells are obtained from the genes for  Aspergillus oryzae  TAKA amylase and  Aspergillus nidulans  triose phosphate isomerase.  
      Suitable leaders for yeast host cells are obtained from the genes for  Saccharomyces cerevisiae  enolase (ENO-1),  Saccharomyces cerevisiae  3-phosphoglycerate kinase,  Saccharomyces cerevisiae  alpha-factor, and  Saccharomyces cerevisiae  alcohol dehydrogenase/glyceraldehyde-3-phosphate dehydrogenase (ADH2/GAP).  
      The control sequence may also be a polyadenylation sequence, a sequence operably linked to the 3′ terminus of the nucleotide sequence and which, when transcribed, is recognized by the host cell as a signal to add polyadenosine residues to transcribed mRNA. Any polyadenylation sequence which is functional in the host cell of choice may be used in the present invention.  
      Preferred polyadenylation sequences for filamentous fungal host cells are obtained from the genes for  Aspergillus oryzae  TAKA amylase,  Aspergillus niger  glucoamylase,  Aspergillus nidulans  anthranilate synthase,  Fusarium oxysporum  trypsin-like protease, and  Aspergillus niger  alpha-glucosidase.  
      Useful polyadenylation sequences for yeast host cells are described by Guo and Sherman, 1995 , Molecular Cellular Biology  15: 5983-5990.  
      The control sequence may also be a signal peptide coding region that codes for an amino acid sequence linked to the amino terminus of an artificial variant of a polypeptide and directs the encoded polypeptide into the cell&#39;s secretory pathway. The 5′ end of the coding sequence of the nucleotide sequence may inherently contain a signal peptide coding region naturally linked in translation reading frame with the segment of the coding region which encodes the secreted polypeptide. Alternatively, the 5′ end of the coding sequence may contain a signal peptide coding region which is foreign to the coding sequence. The foreign signal peptide coding region may be required where the coding sequence does not naturally contain a signal peptide coding region. Alternatively, the foreign signal peptide coding region may simply replace the natural signal peptide coding region in order to enhance secretion of the polypeptide. However, any signal peptide coding region which directs the expressed polypeptide into the secretory pathway of a host cell of choice may be used in the present invention.  
      Effective signal peptide coding regions for bacterial host cells are the signal peptide coding regions obtained from the genes for  Bacillus  NCIB 11837 maltogenic amylase,  Bacillus stearothermophilus  alpha-amylase,  Bacillus licheniformis  subtilisin,  Bacillus licheniformis  beta-lactamase,  Bacillus stearothermophilus  neutral proteases (nprT, nprS, nprM), and  Bacillus subtilis  prsA. Further signal peptides are described by Simonen and Palva, 1993 , Microbiological Reviews  57: 109-137.  
      Effective signal peptide coding regions for filamentous fungal host cells are the signal peptide coding regions obtained from the genes for  Aspergillus oryzae  TAKA amylase,  Aspergillus niger  neutral amylase,  Aspergillus niger  glucoamylase,  Rhizomucor miehei  aspartic proteinase,  Humicola insolens  cellulase,  Humicola insolens  endoglucanase V, and  Humicola lanuginosa  lipase.  
      Useful signal peptides for yeast host cells are obtained from the genes for  Saccharomyces cerevisiae  alpha-factor and  Saccharomyces cerevisiae  invertase. Other useful signal peptide coding regions are described by Romanos et al., 1992, supra.  
      The control sequence may also be a propeptide coding region that codes for an amino acid sequence positioned at the amino terminus of a polypeptide. The resultant polypeptide is known as a proenzyme or propolypeptide (or a zymogen in some cases). A propolypeptide is generally inactive and can be converted to a mature active polypeptide by catalytic or autocatalytic cleavage of the propeptide from the propolypeptide. The propeptide coding region may be obtained from the genes for  Bacillus subtilis  alkaline protease (aprE),  Bacillus subtilis  neutral protease (nprT),  Saccharomyces cerevisiae  alpha-factor,  Rhizomucor miehei  aspartic proteinase, and  Myceliophthora  thermophila laccase (WO 95/33836).  
      Where both signal peptide and propeptide regions are present at the amino terminus of an artificial variant of a polypeptide, the propeptide region is positioned next to the amino terminus of a polypeptide and the signal peptide region is positioned next to the amino terminus of the propeptide region.  
      It may also be desirable to add regulatory sequences which allow the regulation of the expression of the artificial variant of a polypeptide relative to the growth of the host cell. Examples of regulatory systems are those which cause the expression of the gene to be turned on or off in response to a chemical or physical stimulus, including the presence of a regulatory compound. Regulatory systems in prokaryotic systems include the lac, tac, and trp operator systems. In yeast, the ADH2 system or GALL system may be used. In filamentous fungi, the TAKA alpha-amylase promoter,  Aspergillus niger  glucoamylase promoter, and  Aspergillus oryzae  glucoamylase promoter may be used as regulatory sequences. Other examples of regulatory sequences are those which allow for gene amplification. In eukaryotic systems, these include the dihydrofolate reductase gene which is amplified in the presence of methotrexate, and the metallothionein genes which are amplified with heavy metals. In these cases, the nucleotide sequence encoding the polypeptide would be operably linked with the regulatory sequence.  
      Expression Vectors  
      The present invention also relates to recombinant expression vectors comprising a mutant polynucleotide encoding an artificial variant of the present invention, a promoter, and transcriptional and translational stop signals. The various nucleotide and control sequences described above may be joined together to produce a recombinant expression vector which may include one or more convenient restriction sites to allow for insertion or substitution of the nucleotide sequence encoding the artificial variant at such sites. Alternatively, the nucleotide sequence may be expressed by inserting the nucleotide sequence or a nucleic acid construct comprising the sequence into an appropriate vector for expression. In creating the expression vector, the coding sequence is located in the vector so that the coding sequence is operably linked with the appropriate control sequences for expression.  
      The recombinant expression vector may be any vector (e.g., a plasmid or virus) which can be conveniently subjected to recombinant DNA procedures and can bring about expression of the nucleotide sequence. The choice of the vector will typically depend on the compatibility of the vector with the host cell into which the vector is to be introduced. The vectors may be linear or closed circular plasmids.  
      The vector may be an autonomously replicating vector, i.e., a vector which exists as an extrachromosomal entity, the replication of which is independent of chromosomal replication, e.g., a plasmid, an extrachromosomal element, a minichromosome, or an artificial chromosome. The vector may contain any means for assuring self-replication. Alternatively, the vector may be one which, when introduced into the host cell, is integrated into the genome and replicated together with the chromosome(s) into which it has been integrated. Furthermore, a single vector or plasmid or two or more vectors or plasmids which together contain the total DNA to be introduced into the genome of the host cell, or a transposon may be used.  
      The vectors of the present invention preferably contain one or more selectable markers which permit easy selection of transformed, transfected, transduced, or the like cells. A selectable marker, as described earlier, is a gene the product of which provides for biocide or viral resistance, resistance to heavy metals, prototrophy to auxotrophs, and the like.  
      Examples of bacterial selectable markers are the dal genes from  Bacillus subtilis  or  Bacillus licheniformis , or markers which confer antibiotic resistance such as ampicillin, kanamycin, chloramphenicol, or tetracycline resistance. Suitable markers for yeast host cells are ADE2, HIS3, LEU2, LYS2, MET3, TRP1, and URA3. Selectable markers for use in a filamentous fungal host cell include, but are not limited to, amdS (acetamidase), argB (ornithine carbamoyltransferase), bar (phosphinothricin acetyltransferase), hph (hygromycin phosphotransferase), niaD (nitrate reductase), pyrg (orotidine-5′-phosphate decarboxylase), sC (sulfate adenyltransferase), and trpC (anthranilate synthase), as well as equivalents thereof. Preferred for use in an  Aspergillus  cell are the amdS and pyrG genes of  Aspergillus nidulans  or  Aspergillus oryzae  and the bar gene of  Streptomyces hygroscopicus.    
      The vectors of the present invention preferably contain an element(s) that permits integration of the vector into the host cell&#39;s genome or autonomous replication of the vector in the cell independent of the genome.  
      For integration into the host cell genome, the vector may rely on the polynucleotide&#39;s sequence encoding the polypeptide or any other element of the vector for integration into the genome by homologous or nonhomologous recombination. Alternatively, the vector may contain additional nucleotide sequences for directing integration by homologous recombination into the genome of the host cell at a precise location(s) in the chromosome(s). To increase the likelihood of integration at a precise location, the integrational elements should preferably contain a sufficient number of nucleic acids, such as 100 to 10,000 base pairs, preferably 400 to 10,000 base pairs, and most preferably 800 to 10,000 base pairs, which have a high degree of identity with the corresponding target sequence to enhance the probability of homologous recombination. The integrational elements may be any sequence that is homologous with the target sequence in the genome of the host cell. Furthermore, the integrational elements may be non-encoding or encoding nucleotide sequences. On the other hand, the vector may be integrated into the genome of the host cell by non-homologous recombination.  
      For autonomous replication, the vector may further comprise an origin of replication enabling the vector to replicate autonomously in the host cell in question. The origin of replication may be any plasmid replicator mediating autonomous replication which functions in a cell. The term “origin of replication” or “plasmid replicator” is defined herein as a nucleotide sequence that enables a plasmid or vector to replicate in vivo.  
      Examples of bacterial origins of replication are the origins of replication of plasmids pBR322, pUC19, pACYC177, and pACYC184 permitting replication in  E. coli , and pUB110, pE194, pTA1060, and pAMβ1 permitting replication in  Bacillus.    
      Examples of origins of replication for use in a yeast host cell are the 2 micron origin of replication, ARS1, ARS4, the combination of ARS1 and CEN3, and the combination of ARS4 and CEN6.  
      Examples of origins of replication useful in a filamentous fungal cell are AMA1 and ANS1 (Gems et al., 1991 , Gene  98:61-67; Cullen et al., 1987 , Nucleic Acids Research  15: 9163-9175; WO 00/24883). Isolation of the AMA1 gene and construction of plasmids or vectors comprising the gene can be accomplished according to the methods disclosed in WO 00/24883.  
      More than one copy of a mutant polynucleotide of the present invention may be inserted into the host cell to increase production of the gene product. An increase in the copy number of the polynucleotide can be obtained by integrating at least one additional copy of the sequence into the host cell genome or by including an amplifiable selectable marker gene with the polynucleotide where cells containing amplified copies of the selectable marker gene, and thereby additional copies of the polynucleotide, can be selected for by cultivating the cells in the presence of the appropriate selectable agent.  
      The procedures used to ligate the components described above to construct the recombinant expression vectors of the present invention are well known to one skilled in the art (see, e.g., Sambrook et al., 1989, supra).  
      Host Cells  
      The present invention also relates to recombinant host cells, comprising a mutant polynucleotide sequence encoding an artificial variant, which are advantageously used in the recombinant production of the artificial variant. A vector comprising a mutant polynucleotide of the present invention is introduced into a host cell so that the vector is maintained as a chromosomal integrant or as a self-replicating extra-chromosomal vector as described earlier. The term “host cell” encompasses any progeny of a parent cell that is not identical to the parent cell due to mutations that occur during replication. The choice of a host cell will to a large extent depend upon the gene encoding the artificial variant and its source.  
      The host cell may be a unicellular microorganism, e.g., a prokaryote, or a non-unicellular microorganism, e.g., a eukaryote.  
      Useful unicellular microorganisms are bacterial cells such as gram positive bacteria including, but not limited to, a  Bacillus  cell, e.g.,  Bacillus alkalophilus, Bacillus amyloliquefaciens, Bacillus brevis, Bacillus circulans, Bacillus clausii, Bacillus coagulans, Bacillus lautus, Bacillus lentus, Bacillus licheniformis, Bacillus megaterium, Bacillus stearothermophilus, Bacillus subtilis , and  Bacillus thuringiensis ; or a  Streptomyces  cell, e.g.,  Streptomyces lividans  and  Streptomyces murinus , or gram negative bacteria such as  E. coli  and  Pseudomonas  sp. In a preferred aspect, the bacterial host cell is a  Bacillus lentus, Bacillus licheniformis, Bacillus stearothermophilus , or  Bacillus subtilis  cell. In another preferred aspect, the  Bacillus  cell is an alkalophilic  Bacillus.    
      The introduction of a vector into a bacterial host cell may, for instance, be effected by protoplast transformation (see, e.g., Chang and Cohen, 1979 , Molecular General Genetics  168: 111-115), using competent cells (see, e.g., Young and Spizizin, 1961 , Journal of Bacteriology  81: 823-829, or Dubnau and Davidoff-Abelson, 1971 , Journal of Molecular Biology  56: 209-221), electroporation (see, e.g., Shigekawa and Dower, 1988 , Biotechniques  6: 742-751), or conjugation (see, e.g., Koehler and Thorne, 1987 , Journal of Bacteriology  169: 5771-5278).  
      The host cell may also be a eukaryote, such as a mammalian, insect, plant, or fungal cell.  
      In a preferred aspect, the host cell is a fungal cell. “Fungi” as used herein includes the phyla Ascomycota, Basidiomycota, Chytridiomycota, and Zygomycota (as defined by Hawksworth et al., In, Ainsworth and Bisby&#39;s Dictionary of The Fungi, 8th edition, 1995, CAB International, University Press, Cambridge, UK) as well as the Oomycota (as cited in Hawksworth et al., 1995, supra, page 171) and all mitosporic fungi (Hawksworth et al., 1995, supra).  
      In a more preferred aspect, the fungal host cell is a yeast cell. “Yeast” as used herein includes ascosporogenous yeast (Endomycetales), basidiosporogenous yeast, and yeast belonging to the Fungi Imperfecti (Blastomycetes). Since the classification of yeast may change in the future, for the purposes of this invention, yeast shall be defined as described in Biology and Activities of Yeast (Skinner, F. A., Passmore, S. M., and Davenport, R. R., eds, Soc. App. Bacteriol. Symposium Series No. 9, 1980).  
      In an even more preferred aspect, the yeast host cell is a  Candida, Hansenula, Kluyveromyces, Pichia, Saccharomyces, Schizosaccharomyces , or  Yarrowia  cell.  
      In a most preferred aspect, the yeast host cell is a  Saccharomyces carisbergensis, Saccharomyces cerevisiae, Saccharomyces diastaticus, Saccharomyces douglasii, Saccharomyces kluyveri, Saccharomyces norbensis , or  Saccharomyces oviformis  cell. In another most preferred aspect, the yeast host cell is a  Kluyveromyces lactis  cell. In another most preferred aspect, the yeast host cell is a  Yarrowia lipolytica  cell.  
      In another more preferred aspect, the fungal host cell is a filamentous fungal cell. “Filamentous fungi” include all filamentous forms of the subdivision Eumycota and Oomycota (as defined by Hawksworth et al., 1995, supra). The filamentous fungi are generally characterized by a mycelial wall composed of chitin, cellulose, glucan, chitosan, mannan, and other complex polysaccharides. Vegetative growth is by hyphal elongation and carbon catabolism is obligately aerobic. In contrast, vegetative growth by yeasts such as  Saccharomyces cerevisiae  is by budding of a unicellular thallus and carbon catabolism may be fermentative.  
      In an even more preferred aspect, the filamentous fungal host cell is an  Acremonium, Aspergillus, Aureobasidium, Bjerkandera, Ceriporiopsis, Coprinus, Coriolus, Cryptococcus, Filibasidium, Fusarium, Humicola, Magnaporthe, Mucor, Myceliophthora, Neocallimastix, Neurospora, Paecilomyces, Penicillium, Phanerochaete, Phlebia, Piromyces, Pleurotus, Schizophyllum, Talaromyces, Thermoascus, Thielavia, Tolypocladium, Trametes , or  Trichoderma  cell.  
      In a most preferred aspect, the filamentous fungal host cell is an  Aspergillus awamori, Aspergillus fumigatus, Aspergillus foetidus, Aspergillus japonicus, Aspergillus nidulans, Aspergillus niger  or  Aspergillus oryzae  cell. In another most preferred aspect, the filamentous fungal host cell is a  Fusarium bactridioides, Fusarium cerealis, Fusarium crookwellense, Fusarium culmorum, Fusarium graminearum, Fusarium graminum, Fusarium heterosporum, Fusarium negundi, Fusarium oxysporum, Fusarium reticulatum, Fusarium roseum, Fusarium sambucinum, Fusarium sarcochroum, Fusarium sporotrichioides, Fusarium sulphureum, Fusarium torulosum, Fusarium trichothecioides , or  Fusarium venenatum  cell. In another most preferred aspect, the filamentous fungal host cell is a  Bjerkandera adusta, Ceriporiopsis aneirina, Ceriporiopsis aneirina, Ceriporiopsis caregiea, Ceriporiopsis gilvescens, Ceriporiopsis pannocinta, Ceriporiopsis rivulosa, Ceriporiopsis subrufa, Ceriporiopsis subvermispora, Coprinus cinereus, Coriolus hirsutus, Humicola insolens, Humicola lanuginosa, Mucor miehei, Myceliophthora thermophila, Neurospora crassa, Penicillium purpurogenum, Phanerochaete chrysosporium, Phlebia radiata, Pleurotus eryngii, Thielavia terrestris, Trametes villosa, Trametes versicolor, Trichoderma harzianum, Trichoderma koningii, Trichoderma longibrachiatum, Trichoderma reesei , or  Trichoderma viride  cell.  
      Fungal cells may be transformed by a process involving protoplast formation, transformation of the protoplasts, and regeneration of the cell wall in a manner known per se. Suitable procedures for transformation of  Aspergillus  and  Trichoderma  host cells are described in EP 238 023 and Yelton et al., 1984 , Proceedings of the National Academy of Sciences USA  81: 1470-1474. Suitable methods for transforming  Fusarium  species are described by Malardier et al., 1989 , Gene  78: 147-156, and WO 96/00787. Yeast may be transformed using the procedures described by Becker and Guarente, In Abelson, J. N. and Simon, M. I., editors, Guide to  Yeast Genetics and Molecular Biology, Methods in Enzymology , Volume 194, pp 182-187, Academic Press, Inc., New York; Ito et al., 1983 , Journal of Bacteriology  153: 163; and Hinnen et al., 1978 , Proceedings of the National Academy of Sciences USA  75: 1920.  
      Methods of Production  
      The present invention also relates to methods for producing an artificial variant of a parent polypeptide, comprising (a) cultivating a host cell comprising a mutant polynucleotide encoding the variant under conditions conducive for production of the artificial variant, wherein the mutant polynucleotide was obtained by the methods described herein; and (b) recovering the artificial variant.  
      In the production methods of the present invention, the cells are cultivated in a nutrient medium suitable for production of the artificial variant using methods well known in the art. For example, the cell may be cultivated by shake flask cultivation, and small-scale or large-scale fermentation (including continuous, batch, fed-batch, or solid state fermentations) in laboratory or industrial fermentors performed in a suitable medium and under conditions allowing the polypeptide to be expressed and/or isolated. The cultivation takes place in a suitable nutrient medium comprising carbon and nitrogen sources and inorganic salts, using procedures known in the art. Suitable media are available from commercial suppliers or may be prepared according to published compositions (e.g., in catalogues of the American Type Culture Collection). If the artificial variant is secreted into the nutrient medium, the polypeptide can be recovered directly from the medium. If the artificial variant is not secreted, it can be recovered from cell lysates.  
      The artificial variants may be detected using methods known in the art that are specific for the variants. These detection methods may include use of specific antibodies, formation of an enzyme product, or disappearance of an enzyme substrate. For example, an enzyme assay may be used to determine the activity of the polypeptide as described herein. A multiplicity of assays are available and known in the art. For examples see Manual of Methods for General Bacteriology (Phillipp Gerhardt, R. G. E. Murray, Ralph N. Costilow, Eugene W. Nester, Willis A. Wood, Noel R. Krieg and G. Briggs Phillips, eds), American Society for Microbiology, Washington, D.C. (1994)) or by Thomas D. Brock in Biotechnology: A Textbook of Industrial Microbiology, Second Edition, Sinauer Associates, Inc., Sunderland, Mass. (1989).  
      The resulting artificial variant may be recovered using methods known in the art. For example, the variant may be recovered from the nutrient medium by conventional procedures including, but not limited to, centrifugation, filtration, extraction, spray-drying, evaporation, or precipitation.  
      The artificial variants of the present invention may be purified by a variety of procedures known in the art including, but not limited to, chromatography (e.g., ion exchange, affinity, hydrophobic, chromatofocusing, and size exclusion), electrophoretic procedures (e.g., preparative isoelectric focusing), differential solubility (e.g., ammonium sulfate precipitation), SDS-PAGE, or extraction (see, e.g.,  Protein Purification , J.-C. Janson and Lars Ryden, editors, VCH Publishers, New York, 1989) to obtain substantially pure variants.  
      The present invention is further described by the following examples which should not be construed as limiting the scope of the invention.  
     EXAMPLES  
      Yeast strain  Saccharomyces cerevisiae  JG169 (MATα, ura3-52, leu2-3, pep4-1137, his3Δ2, prb1::leu2, and Δpre1::his3) was used for expression of the beta-glucosidase random insertional library.  
     Example 1  
     Construction of pSATe111  Saccharomyces cerevisiae  Expression Vector  
      A 2,605 bp DNA fragment comprising the region from the ATG start codon to the TAA stop codon of an  Aspergillus oryzae  beta-glucosidase coding sequence (SEQ ID NO: 1 for cDNA sequence and SEQ ID NO: 2 for the deduced amino acid sequence) was amplified by PCR from pJaL660 (WO 2002/095014) as template with primers 992127 (sense) and 992328 (antisense) shown below.  
                              992127:               5′-       GCAGATCTACCATGAAGCTTGGTTGGATCGAG-3′   (SEQ ID NO: 3)               992328:       5′-       GCCTCGAGTTACTGGGCCTTAGGCAGCGAG-3′   (SEQ ID NO: 4)          
 
 Primer 992127 has an upstream Bgl II site and primer 992328 has a downstream Xho I site. 
 
      The amplification reactions (50 μl) were composed of 1×PCR buffer containing MgCl 2  (Roche Applied Science, Manheim, Germany), 0.25 mM dNTPs, 50 μM primer 992127, 50 μM primer 992328, 80 ng of pJaL660, and 2.5 units of Pwo DNA Polymerase (Roche Applied Science, Manheim, Germany). The reactions were incubated in an Eppendorf Mastercycler 5333 (Eppendorf Scientific, Inc., Westbury, N.Y.) programmed for 1 cycle at 94° C. for 5 minutes followed by 25 cycles each at 94° C. for 60 seconds, 55° C. for 60 seconds, and 72° C. for 120 seconds (10 minute final extension).  
      The PCR product was then subcloned into the PCR-Blunt II-TOPO vector using the PCR-Blunt II-TOPO Cloning Kit (Invitrogen, Carlsbad, Calif.) following the manufacturer&#39;s instructions to generate pSATe101 ( FIG. 1 ). Plasmid pSATe101 was digested with Bgl II and Xho I to liberate the beta-glucosidase gene. The reaction products were isolated on a 1.0% agarose gel using 40 mM Tris-acetate-1 mM EDTA (TAE) buffer where a 2.6 kb product band was excised from the gel and purified using a QlAquick Gel Extraction Kit (QIAGEN Inc., Valencia, Calif.) according to the manufacturer&#39;s instructions.  
      The 2.6 kb PCR product was digested and cloned into the Bam HI and Xho I sites of the copper inducible 2 μm yeast expression vector pCu426 (Labbe and Thiele, 1999 , Methods Enzymol.  306: 145-53) to generate pSATe111 ( FIG. 2 ).  
     Example 2  
     Construction of  Aspergillus oryzae  Beta-Glucosidase Entry Vector  
      The  Aspergillus oryzae  beta-glucosidase gene was amplified by PCR using plasmid pSATe111 as a template. The following primers were used to amplify the beta-glucosidase gene with the desired restriction sites (the restriction recognition sites are italicized and the beta-glucosidase coding sequence is underlined). Forward primer Jal660_BG_Sal1_F:  
                          (SEQ ID NO: 5)                                 5′-GCACGC GTCGAC A CC ATGAAGCTTGGTTGGATCGAG -3′              
 
      Reverse primer aBGXho.1A  
                                          5′- GATGCACATGA CTCGAG TTACTGG -3′   (SEQ ID NO: 6)              
 
      The amplification reactions (50 μl) were composed of 1×PCR buffer containing MgCl 2 , 0.2 mM dNTPs, 50 pM each primer, 50 ng of pSATE111, and 2.5 units of Herculase DNA Polymerase (Stratagene Inc., La Jolle, Calif.). The reactions were incubated in an Eppendorf Mastercycler 5333 programmed for 1 cycle at 95° C. for 3 minutes followed by 30 cycles each at 95° C. for 30 seconds, 55° C. for 30 seconds, and 72° C. for 90 seconds (5 minute final extension).  
      The PCR product (approximately 2.6 kb) was purified using a MiniElute™ Kit (QIAGEN Inc., Valencia, Calif.) according to the manufacture&#39;s instructions.  
      The PCR product was restriction digested with Sal I and Xho I and ligated into pENTR 1A (Invitrogen, Carlsbad, Calif.) which was also digested with Sal I and Xho I to generate pAJF-1 ( FIG. 3 ). The ligation reaction was carried out using a Rapid Ligation Kit (Roche Applied Science, Manheim, Germany). Plasmid pAJF-1 contains a kanamycin resistance gene, a pUC origin of replication for maintenance in  E. coli , and two att sites flanking the beta-glucosidase gene for LR Clonase™ mediated Gateway recombination.  
     Example 3  
     Construction of an  Aspergillus oryzae  Beta-Glucosidase Destination Vector  
      The entry vector pAJF-1 containing the  Aspergillus oryzae  beta-glucosidase gene was used to generate the destination vector pAJF-2 through recombination with plasmid pYESDEST-52 (Invitrogen, Carlsbad, Calif.) mediated by Gateway LR Clonase™ (Invitrogen, Carlsbad, Calif.) according to the manufacturer&#39;s instructions. The Gateway LR recombination reaction (20 μl) was composed of 300 ng of pAJF-1, 300 ng pYESDEST-52, 1× reaction buffer (Invitrogen, Carlsbad, Calif.), and 4 μl of LR Clonase™. The reaction was incubated for 21 hours at 25° C. Proteinase K (2 μg/μl) was added and the reaction was incubated for 10 minutes at 37° C. An aliquot (1 μl) from this reaction was used to transform  E. coli  Top 10 competent cells (Invitrogen, Carlsbad, Calif.). Ampicillin selection and sequence analysis of a colony isolate confirmed proper insertion of the  Aspergillus oryzae  beta-glucosidase gene in pYESDEST-52. This plasmid, identified as pAJF-2 ( FIG. 4 ), contains the GAL1 promoter for inducible gene expression in  Saccharomyces cerevisiae , the beta-lactamase gene coding for ampicillin resistance in  E. coli , the pUC ori for replication in  E. coli , the URA3  Saccharomyces cerevisiae  auxotrophic selection marker, and the  Saccharomyces cerevisiae  2μ origin of replication. Plasmid pAJF-2 was used as a wild-type control for comparison with pAJF-2 transposon insertion libraries.  
     Example 4  
     Random Insertional Library Generation  
      The Entranceposon M1-Cam® (Finnzymes Oy, Espoo, Finland) and the Mutation Generation System™ (MGS™, Finnzymes Oy, Espoo, Finland) were used to generate transposon insertions in plasmid pAJF-1 according to the manufacturer&#39;s instructions.  
      The Entranceposon M1-Cam® utilizes the bacteriophage Mu transposase to insert an artificial transposon at random positions within a target DNA population (Mizuuchi, 1992 , Annual Review of Biochemistry  61: 1011-1051; Haapa et al., 1999 , Nucleic Acids Research  27: 2727-2784). The artificial 1.254 kb transposon used in this system contains the following components: 44 bp 5′ and 3′ conserved tandem inverted repeats which act as recognition sites for the Mu transposase, Not I sites located within the inverted repeats that are used for transposon removal and self-ligation, and internal to these repeats is the coding sequence for a chloramphenicol selection marker. After insertion, the transposon can subsequently be removed using the restriction enzyme Not I followed by self-ligation of the backbone which results in a 15 bp in-frame insertion. Ten of 15 bps inserted originate from the inverted repeat sequence that flanks the transposon. The other 5 bp are a result of duplication of the target site that occurs upon integration. The five amino acid insert can be translated into three different peptide combinations based on the insertion frame. In one frame three of the five amino acids are alanines, which is a desired outcome for less deleterious changes to the overall structure of a protein.  
      Five different transposition reactions were performed with the following modifications from the Finnzymes protocol: (1) 200 ng of pAJF-1; (2) 100 ng of pAJF-1; (3) 100 ng of pAJF-1, 2 μl of MuA transposase (Finnzymes Oy, Espoo, Finland), and incubated at 30° C. for 2 hours; (4) 1 μg of pAJF-1; and (5) 1 μg of pAJF-1, 2 μl of MuA transposase, and incubated at 30° C. for 2 hours. Each reaction (20 μl) consisted of the indicated quantity of DNA, 1×MuA transposase buffer, 100 ng of Entranceposon M1-Cam™, and 1 μl of MuA transposase (Finnzymes Oy, Espoo, Finland). The reactions were incubated for 1 hour at 30° C. and then 10 minutes at 75° C.  
      Competent  E. coli  Top10 cells (Invitrogen, Carlsbad, Calif.) were transformed with 5 μl of each of the transposition reactions. Transformants were selected on LB agar plates supplemented with 50 μg of kanamycin per ml and 10 μg of chloramphenicol per ml grown overnight at 37° C. The resistant colonies were rinsed off the plates and DNA was isolated using a Plasmid Midi-Prep Kit (QIAGEN Inc., Valencia, Calif.). Five separate libraries were generated from the five different transposon reactions.  
      Approximately 20,000 pAJF-1 clones containing a transposon in the plasmid were isolated from the five transposon reactions using dual antibiotic selection (i.e., the entry vector encodes kanamycin resistance and the transposon chloramphenicol).  
      Following transposon mutagenesis, the mutated beta-glucosidase genes from the transposon-containing entry vector library were transferred to the Gateway yeast destination vector pYESDEST-52. LR Clonase™ was used to carry out the Gateway transfer reaction according to the manufacturer&#39;s instructions with the following modifications: 300 ng of destination vector, 300 ng of entry vector, and the reaction time was extended to 21 or 25 hours.  
      Competent  E. coli  Top10 cells were transformed with 1 to 2 μl of the Gateway reaction. Transformants were selected on LB agar plates supplemented with 100 μg of ampicillin per ml and 10 μg of chloramphenicol per ml grown overnight at 37° C. Resistant colonies were rinsed off the plates and DNA was isolated using a QIAGEN Plasmid Midi-Prep Kit. Approximately 26,000 clones were isolated. A small portion of the transformation was also plated onto LB agar plates supplemented with 100 μg of ampicillin per ml. A portion of these colonies were then patched onto LB agar plates supplemented with 100 μg of ampicillin per ml and 10 μg of chloramphenicol per ml to determine the approximate number of pENTR1A clones containing a transposon located outside of the beta-glucosidase coding region. As a negative control the Gateway reaction was carried out without the entry vector to determine the ampicillin resistant background generated from the destination vector.  
      The results showed that between 43 and 67% of the clones subjected to transposon mutagenesis contained a gene-directed insertion, representing about 10,000 clones. The negative control reaction showed that only three colonies were ampicillin resistant, resulting in a very low background of vector alone from the Gateway reaction.  
      The inserted transposon was subsequently removed from the library to leave a 15 bp insertion. This was accomplished by collecting library colonies into a single pool and utilizing a QlAfilter Midi Plasmid Kit (QIAGEN Inc., Valencia, Calif.) to isolate library plasmid DNA. The restriction endonuclease Not I was utilized to excise the Mu transposase recognition sites and the chloramphenicol selectable marker. Agarose gel (0.8%) electrophoresis using TAE buffer was used to identify the library plasmid void of the artificial transposon. This backbone fragment was gel purified using a QiaQuick Gel Purification Kit (QIAGEN Inc., Valencia, Calif.) and religated using a Rapid Ligation Kit (Roche Applied Science, Manheim, Germany) according to the manufacturer&#39;s instructions with the following modifications: 100 ng or 20 ng of vector DNA was used, and the reaction time was extended to 30 minutes at 16° C. Competent  E. coli  Top10 cells were transformed with 5 μl of the ligation reaction.  
      Transformants were selected on LB agar plates supplemented with 100 μg of ampicillin per ml grown overnight at 37° C. Approximately, 66,000 clones were isolated, representing 10,000 independent insertion events. From this library, 96 resistant clones were patched onto LB agar plates supplemented with 100 μg of ampicillin per ml and 10 μg of chloramphenicol per ml to obtain an estimate of the number of clones containing the full transposon insert. Only 1 transformant survived dual selection, suggesting that less than 2% of the library contained the full transposon insertion.  
      For characterization and sequencing purposes, the 50 ampicillin resistant colonies were grown overnight in LB medium and DNA was obtained using a QIAGEN QlAfilter Midi Plasmid Kit.  
     Example 5  
     Random Insertional Library Characterization  
      The beta-glucosidase insertional mutants from the final transposon libraries after the transposon was removed were sequenced to determine the position and type of insertion resulting. DNA sequencing was performed on an AB13700 (Applied Biosystems, Foster City, Calif.) using dye terminator chemistry (Giesecke et al., 1992 , Journal of Virol. Methods  38: 47-60). Sequences were assembled using phred/phrap/consed (University of Washington, Seattle Wash.) with sequence specific primers. Fifty clones were sequenced, revealing that 47 (94%) of the clones contained inserts, 2 (4%) lacked inserts, and 1 (2%) contained the entire transposon. Of the 47 clones with inserts, 3 of them had only 14 bp inserts resulting in frame shift mutations. All three of these mutants had the same deletion in the 10 bp sequence that is left from the transposon inverted repeat sequence. Of the 47 clones with inserts, 41 clones were unique. Eleven clones in total resulted from identical insertions at 5 different sites ( FIG. 5 ). However, there were no obvious hot spots where preferential insertion seemed to be occurring. The 15 bp insert can result in different amino acid combinations based on the frame of insertion. Based on the 41 unique clones, 16 (39%) of the inserts occurred in the first frame, 14 (34%) in the second, and 11 (27%) in the third.  
     Example 6  
     Expression of the Beta-Glucosidase Random Insertional Library in  Saccharomyces cerevisiae    
      To study the beta-glucosidase phenotype of the 41 variants containing inserts described in Example 5, plasmid DNA from all 41 variants was used to transform  Saccharomyces cerevisiae  JG169. The YeastMaker Yeast Transformation System 2 (Clontech Laboratories, Inc., Palo Alto, Calif.) was used for transformation according to the manufactures instructions.  
      Selection and induction of the beta-glucosidase insertional mutant transformants was accomplished by plating the transformation on galactose induction medium. Galactose induction medium was composed per liter of 6.7 g of yeast nitrogen base with ammonium sulfate, 5 g of casamino acids, 20 g of agar, and 100 ml of 0.5 M sodium succinate pH 5.0, brought to 860 ml with deionized water, autoclaved for 25 minutes, and cooled to 55° C. After cooling, the following filter sterilized supplements were added: 40 ml of 50% glucose (final 2%), 100 ml of 20% D(+)-galactose (final 2%), and 0.2 ml of 500 mg/ml 5-bromo-4-chloro-3-indolyl-beta-D-glucopyranoside (X-gic) (final 100 mg/l) in DMSO (final 0.02% vol/vol). Yeast colonies were grown for 3 to 5 days at 30° C. Colonies producing active beta-glucosidase turned blue after incubation due to beta-glucosidase hydrolysis of X-glc. Qualitative beta-glucosidase activity was estimated by visual intensity of the blue color and size of the colony.  
      The beta-glucosidase activity for these clones fell into 7 color/size categories: dark blue (tiny colonies, WT like) 13%, dark blue (medium sized) 10%, blue (medium sized), light blue (medium sized) 19%, very light blue (medium sized) 4%, mixture of white and blue 4%, and no color 38%. These phenotypes were matched on the insertion distribution map ( FIG. 6 ).  
     Example 7  
     Codon Triplet Substitution—Using Bsg I and Btg ZI  
      A polypeptide encoding a substitution variant of the glucoamylase from  Talaromyces emersonii , T-AMG, was constructed according to the present invention. The experiments performed are outlined below:  
      (1) Transposons with kanamycin resistance were inserted into plasmid pMiBg235 yielding libraries of about 1×10 6  transformants.  
      (2) Experiments where transformants were plated out on either ampicillin or kanamycin plates showed 100 times more colonies on ampicillin plates, which indicated a high probability for only one transposon per gene.  
      (3) Plasmid preparations of pooled transformants showed that only DNA with the gene coding for kanamycin resistance was obtained.  
      (4) Restriction with enzymes flanking the gene of interest yielded four strong bands on agarose gels: a fragment containing the gene, gene with transposon, vector minus gene, and vector minus gene with transposon.  
      (5) The cloning steps showed relatively high transformation rates between 600,000 to 12×10 6  transformants.  
      (6) Sequence analysis of resulting plasmids from each cloning step showed the expected restrictions and finally the desired substitutions (see below).  
      DNA fragment manufacture. Enzymes and a transposon kit (‘Mutation Generation System’) were purchased from Finnzymes Oy, Espoo, Finland, ‘PCR Polishing Kit’ was from Stratagene Corp., La Jolla, Calif., and oligos were obtained from DNA Technology, Arhus, Denmark.  
      Two oligos were designed with various restriction sites (see  FIG. 7A  for details). tcgagatcgaacagcggccgcatcgcagctggcaggtacggatcgatcctagtaagcca (SEQ ID NO: 13) acgatcgagctcagcggccgcatctgcacgtgcagctaaggcagtcgagctnnntcgagcaggtcggatgatccagttcgatttattc (SEQ ID NO: 17)  
      A Not I-Not I DNA fragment was synthezised by PCR with the designed oligos using the commercial transposon ENTRANCEPOSON™ (Finnzymes Oy, Espoo, Finland) (M1-Kanamycin) as template (the sequence of the transposon is shown in SEQ ID NO: 9). To achieve high transformation rates, the synthesized fragment with the outside cutter recognition sites and the three random or partially random base pairs ‘NNN’ (N indicates 25% of T, C, G, and A) was first subcloned (6,400 transformants). Subsequently, the fragment was introduced into the inserted transposon in the gene of interest, replacing most of the inserted transposon in the process.  
      Cloning of T-AMG gene. Plasmid pStep202 is an episomal expression vector based on the very well-known inducible yeast expression vector pYES2, wherein the gal4 promoter of pYES2 was replaced by a constitutive triose phophate isomerase (TPI) promoter, using standard procedures. The TPI promoter ensures constitutive expression of the gene, when the gene of interest is cloned downstream the TPI promoter. The vector comprises the URA3 marker, a gene of the synthetic pathway for uracil, encoding oritidine 5′-decarboxylase which allows for selection on minimal medium. The vector further contains the 2My origin of DNA replication. An ampicillin resistance gene is conveniently used for selection in  E. coli.    
      The cDNA of the T-AMG gene encoding the amyloglucosidase from  Talaromyces emersonii  was cloned into the yeast/ E. coli  shuttle vector pStep202 as a HindIII/XbaI PCR fragment to yield the vector pStep226. pStep202 is derived from the yeast expression vector pYES 2.0 (Invitrogen, UK and Kofod et al., 1994 , J. Biol. Chem.  269: 29182-29189). Both pStep202 and pStep226 replicate in  E. coli  and  S. cerevisiae.    
      Plasmid pMiBg235 is identical to pStep226, except that one Bfu AI restriction site and three Btg ZI restriction sites present in pStep226 have been removed to facilitate the use of these ‘outside cutting’ restriction enzymes in the cloning steps of the invention.  
      Insertion of transposon. The Finnzymes ‘Mutation Generation System’ kit was used for random insertion of a transposon into pMiBg235, which contains the gene coding for an amyloglucosidase (AMG) from a Talaromyces species, denoted T-AMG. Three hundred and ten ng of pMiBg235 were mixed with 100 ng of Entranceposon (M1-Kanamycin) (Finnzymes Oy, Espoo, Finland), 1 μl of MuA transposase, and 4 μl of the manufacturer&#39;s 5×MuA reaction buffer in a total volume of 20 μl. The transposition reaction was allowed to proceed for 60 minutes at 30° C. and the MuA transposase was subsequently inhibited by incubation at 75° C. for 10 minutes.  
      Plasmid DNA was isolated and purified into a volume of 15 μl, 1 or 3 μl thereof was then electrotransformed into competent  E. coli  cells according to standard procedures, and transformants were spread out onto LB plates supplemented with 10 μg/ml kanamycin to yield 16,000 and 65,000 kanamycin resistant transformants, respectively. The procedure was repeated to yield a total number of about 1×10 6  transformants. Transposon containing plasmid DNA was purified from overnight incubations of selected transformed  E. coli  cells in LB medium supplemented with 100 μg of ampicillin and 10 μg of kanamycin per ml.  
      Isolation of T-AMG genes with transposons. In order to isolate T-AMG genes with transposons, 10 μg of plasmid was restricted with Pac I and Xba I, which should result in four DNA fragments: the original vector, the T-AMG gene fragment, plus vector- and T-AMG gene fragments with inserted transposon. The T-AMG gene DNA fragment with the transposon inserted was isolated by agarose gel electrophoresis and cloned back into Pac I and Xba I digested pMiBg235 vector; 600,000 kanamycin-resistant transformants were obtained.  
      Introduction of DNA fragment with outside cutter sites. DNA-fragments flanked with outside cutters ( FIG. 7 ) were cloned into the library of T-AMG genes (with transposons) using the two flanking Not I-sites of the inserted transposon: 10 μg of plasmid DNA of the T-AMG (with transposons) was digested with Not I and the vector and T-AMG fragments were isolated from the transposon fragment and ligated to the Not I restricted PCR-fragments; 600,000 kanamycin-resistant transformants were obtained.  
      Trimming flanking site by Bsq I restriction. A fragment containing one of the Not I-sites and parts of the neighboring duplicated target site was digested from the construct with Bsg I and the vector/T-AMG DNA-fragment purified on an agarose gel. The remaining sticky-ends were blunt-ended by PCR polishing, removing all five base pairs in the duplicated target site. The three random or partially random base pairs were brought next to the coding sequence of T-AMG by ligation of the two blunt ends of the vector/T-AMG DNA-fragment. The circularized vector was then transformed into  E. coli  yielding 5.6×10 6  transformants.  
      Trimming flanking site by Btq ZI and Pvu II restriction. A fragment containing one of the NotI-sites and parts of the neighboring duplicated target site was digested with Btg ZI and Pvu II, and the vector/T-AMG DNA-fragments were isolated from an agarose gel. The remaining sticky-ends were blunt-ended by PCR polishing by filling in basepair 1 and 2 of the duplicated target site. A Bfu AI site was brought into a position close to the coding sequence of T-AMG by subsequent ligation of the two blunt ends of the vector/T-AMG DNA-fragment. The circulated vector was transformed into  E. coli  yielding 8×10 5  transformants.  
      Excision of transposon by Bfu AI restriction. The remaining fragment was excised by digestion with Bfu AI and the linearized vector was purified from an agarose gel. The sticky-ends were then PCR polished and the vector was relegated. The position of the Bfu AI site with respect to basepair 1 and 2 of the duplicated target site was designed so that the Bfu AI restriction in this step would bring the random or partially random codon-triplet ‘NNN’ into position next to base pair 1 and 2 after the religation, thereby replacing base pair 3, 4 and 5 of the duplicated target site. The circularized vector was transformed into  E. coli  yielding 12×10 6  transformants.  
      Sequence analysis. DNA-sequence analysis of three different resulting variants of the  Talaromyces  amyloglucosidase yielded the following amino acid substitutions:  
                                                          Variant 1: Q82W   Position:       80   81   82   83                   Amino acid sequence of wt:   N-term.       I   Q   Q   Y       C-term.       Coding sequence of wt:       5′   ATC   CAG   CAG   TAC   3′       Coding sequence of variant 1:       5′   ATC   CAA   TGG   TAC   3′           N-term.       I   Q   W   Y       C-term.               Variant 2: Q81G   Position:       80   81   82   83       Amino acid sequence of wt:   N-term.       I   Q   Q   Y       C-term.       Coding sequence of wt:       5′   ATC   CAG   CAG   TAC   3′       Coding sequence of variant 2:       5′   ATA   GGG   CAG   TAC   3′           N-term.       I   G   W   Y       C-term.                                                         Variant 3: S165P   Position:       164   165   166                   Amino acid sequence of wt:   N-term.       L   S   Y       C-term.       Coding sequence of wt:       5′   CTG   TCC   TAC   3′       Coding sequence of variant 3:       5′   CTG   CCT   TAC   3′           N-term.       L   P   Y       C-term.          
 
     Example 8  
     Codon Triplet Substitution Using Bsg I and Acu I  
      A polypeptide encoding a substitution variant of a maltogenic amylase from  Bacillus stearothermophilus  was constructed according to the present invention. The experiments performed are outlined below:  
      (1) Transposons with kanamycin resistance were inserted into plasmid pMiBg242 yielding libraries of about 1×10 6  transformants.  
      (2) Experiments where transformants were plated out on either ampicillin or kanamycin plates showed 100 times more colonies on ampicillin plates, which indicated a high probability for only one transposon per gene.  
      (3) Plasmid preparations of pooled transformants showed that only DNA with the gene coding for kanamycin resistance was obtained.  
      (4) Restriction with enzymes flanking the gene of interest yielded four strong bands on agarose gels: a fragment containing the gene, gene with transposon, vector minus gene, and vector minus gene with transposon.  
      (5) The cloning steps showed relatively high transformation rates between 600.000 to 12×10 6  transformants.  
      (6) Sequence analysis of resulting plasmids from each cloning step showed the expected restrictions and finally the wanted substitutions (see text below).  
      DNA fragment manufacture. Enzymes and transposon kit (‘Mutation Generation System’) were purchased from Finnzymes Oy, Espoo, Finland, ‘PCR Polishing Kit’ was from Stratagene Corp., La Jolla, Calif., and oligos were obtained from DNA Technology, Arhus, Denmark.  
      Two oligos were designed with various restriction sites (see  FIG. 8A ):  
                              atcgagctcagcggccgcttctgcacccaattggttnnncgtccaagtggctgcacttcagcggatgatccagttcgatttattc   (SEQ ID NO:18)                   tcgagatcgaacagcggccgctggacttcagacggatcgatcctagtaagcca   (SEQ ID NO:12)          
 
      A PCR-fragment was synthesized with the designed oligos using the commercial transposon ENTRANCEPOSON™ (M1-Kanamycin) as template (the sequence of the transposon is shown in SEQ ID NO: 9). To achieve high transformation rates, the synthesized fragment with the outside cutter recognition sites and the three random or partially random base pairs ‘NNN’ (N indicates 25% of T, C, G and A) was first subcloned (6,400 transformants). Subsequently, the Not 1-digested PCR-fragment was introduced into the Not I-sites of the previously inserted transposon in the gene of interest, effectively replacing most of the inserted transposon in the process (see  FIG. 8B ).  
      Cloning of T-AMG gene. The Acu I sites of pMiBg235 vector described above were removed to yield the vector pMiBg231 to facilitate the use of this ‘outside cutting’ restriction enzyme in the cloning steps of the invention. The cDNA of a gene encoding a maltogenic amylase from  Bacillus stearothermophilus  was cloned into the yeast/ E. coli  shuttle vector pMiBg231 as a Pac l/Xba I PCR fragment without Acu I sites to yield the vector pMiBg242.  
      Insertion of transposon. The Finnzymes ‘Mutation Generation System’ kit was used for random insertion of a transposon into plasmid DNA containing the gene coding for the maltogenic amylase. A total of 310 ng of pMiBg242 was mixed with 100 ng of Entranceposon (M1-Kanamycin), 1 μl of MuA transposase, and 4 μl of the manufacturer&#39;s 5×MuA reaction buffer in a total volume of 20 μl. The transposition reaction was allowed to proceed for 60 minutes at 30° C. and the MuA transposase was subsequently inhibited by incubation at 75° C. for 10 minutes.  
      Plasmid DNA was isolated and purified into a volume of 15 μl, 1 or 3 μl thereof was then electrotransformed into competent  E. coli  cells according to standard procedures, and transformants were spread out on LB-kanamycin plates (10 μg/ml) yielding 16,000 and 65,000 kanamycin resistent transformants, respectively. The procedure was repeated yielding a total number of about 1×10 6  transformants. Transposon containing plasmid DNA was purified from overnight incubations of selected transformed  E. coli  cells in LB-ampicillin (100 μg/ml) and kanamycin (10 μg/ml) medium.  
      Isolation of genes with transposons. In order to isolate genes with transposons, 10 μg of plasmid was restricted with Pac I and Xba 1, which should result in four DNA fragments: the original vector, the gene fragment, plus vector- and gene fragments with inserted transposon. The maltogenic amylase encoding gene fragment with the transposon inserted was isolated by agarose gel electrophoresis and cloned back into Pac I and Xba I restricted pMiBg242 vector. More than 500,000 kanamycin-resistant transformants were obtained.  
      Introduction of DNA fragment with outside cutter sites. Not I-digested DNA-fragments flanked with outside cutters were introduced into the library of maltogenic amylase genes (with transposons) in the two flanking Not I-sites of the inserted transposon: 10 μg of plasmid DNA of the amylase encoding gene (with transposons) was cut with Not I and the vector- and gene-fragments were isolated from the transposon fragment and ligated to the Not I restricted PCR-fragments. More than 500,000 kanamycin-resistant transformants were obtained.  
      Trimming flanking site by Bsq I restriction. A fragment containing one of the Not I-sites and parts of the neighbouring duplicated target site was digested from the construct with Bsg I and the vector/gene-fragment purified on agarose gel. The remaining sticky-ends were blunt-ended by PCR polishing, removing all five base pairs in the duplicated target site. The three random or partially random base pairs were brought next to the coding sequence of the maltogenic amylase gene by ligation of the two blunt ends of the vector/gene-fragment. The circularized vector was then transformed into  E. coli  yielding more than 1×10 6  transformants.  
      Trimming flanking site and excision of transposon by Acu I restriction. The remaining transposon fragment was excised by restriction with Acu I of two Acu I sites at each end of the inserted transposon and the linearized vector was purified from an agarose gel. The sticky-ends were then PCR polished and the vector was religated. The design of the position of one of the Acu I sites with respect to basepair 1 and 2 of the duplicated target site was done so that the Acu I restriction in this step would bring the random or partially random codon-triplet ‘NNN’ into position next to base pair 1 and 2 after the religation, thereby replacing base pair 3, 4 and 5 of the duplicated target site. The circularized vector was transformed into  E. coli  yielding more than 1×10 6  transformants.  
      Sequence analysis. DNA-sequence analysis of three different resulting variants gave following amino acid substitutions:  
                                                      Variant 1: D326T   Position:       325   326   327                   Amino acid sequence of wt:   N-term.       I   D   N       C-term.       Coding sequence of wt:       5′   ATC   GAT   AAC   3′       Coding sequence of variant 1:       5′   ATA   ACT   AAC   3′           N-term.       I   T   N       C-term.               Variant 2: K340I   Position:       339   340   341       Amino acid sequence of wt:   N-term.       N   K   A       C-term.       Coding sequence of wt:       5′   AAC   AAG   GCG   3′       Coding sequence of variant 2:       5′   AAC   ATC   GCG   3′           N-term.       N   I   A       C-term.          
 
     Example 9  
     Codon Triplet Deletion  
      A polypeptide encoding a deletion variant of a maltogenic amylase from  Bacillus stearothermophilus  was constructed according to the present invention. The experiments showed that it was possible to insert a transposon into the gene of interest and that transposon could be excised to provide one, two or three deleted codon triplets in the gene. The experiments performed are outlined below:  
      (1) Transposons with kanamycin resistance were inserted into plasmid pMiBg242 yielding libraries of about 1×10 6  transformants.  
      (2) Experiments where transformants were plated out on either ampicillin or kanamycin plates showed 100 times more colonies on ampicillin plates, which indicated a high probability for only one transposon per gene.  
      (3) Plasmid preparations of pooled transformants showed that only DNA with the gene coding for kanamycin resistance was obtained.  
      (4) Restriction with enzymes flanking the gene of interest yielded four strong bands on agarose gels: a fragment containing the gene, gene with transposon, vector minus gene and vector minus gene with transposon.  
      (5) The cloning steps showed relatively high transformation rates between 600.000 to 12×10 6  transformants.  
      (6) Sequence analysis of resulting plasmids from each cloning step showed the expected restrictions and finally the wanted deletions (see text below).  
      DNA fragment manufacture. Enzymes and a transposon kit (‘Mutation Generation System’) were purchased from Finnzymes Oy, Espoo, Finland, ‘PCR Polishing Kit’ was from Stratagene Corp., La Jolla, Calif., and oligos were obtained from DNA Technology, Arhus, Denmark.  
      Two oligos to obtain one deleted codon triplets were designed with various restriction sites (see  FIG. 9A  for details):  
                          (SEQ ID NO:19)                                 atcgagctcagcggccgcctgcaccggatgatccagttcgatttattc                                     (SEQ ID NO:15)                                 tcgagatcgaacagcggccgcaaggaactgcacacggatcgatcctagtaagcca              
 
      To obtain two or three deleted codon triplets instead of just one, two oligos were designed with various restriction sites to replace SEQ ID NO 9, respectively, in the following strategy:  
      Two Deleted Codon Triplets:  
                          (SEQ ID NO:20)                                 tcgagatcgaacagcggccgcaagctgcacacggatcgatcctagtaagcca              
 
      Three Deleted Codon Triplets:  
                          (SEQ ID NO:21)                         tcgagatcgaacagcggccgcctgcacacggatcgatcctagtaagcca              
 
      A Not I-Not I DNA fragment was synthesized by PCR with the designed oligos using the commercial transposon ENTRANCEPOSON™ as template (the sequence of the transposon is shown in SEQ ID NO: 9). To achieve high transformation rates, the synthesized fragment with the outside cutter recognition sites was first subcloned (7,000 transformants). Subsequently, the fragment was cloned into the inserted transposon in the gene of interest, replacing most of the inserted transposon in the process.  
      Cloning of amylase gene. The Acu I sites of pMiBg235 described above were removed to yield the vector pMiBg231 to facilitate the use of these ‘outside cutting’ restriction enzymes in the cloning steps of the invention. The cDNA of the gene encoding the maltogenic amylase from  Bacillus stearothermophilus  was cloned into the yeast/ E. coli  shuttle vector pMiBg231 as a Pac I/Xba I PCR fragment without Acu I sites to yield the vector pMiBg242.  
      Insertion of transposon. The Finnzymes ‘Mutation Generation System’ kit was used for random insertion of transposon into plasmid DNA containing the gene coding for the maltogenic amylase. A total of 310 ng of pMiBg242 was mixed with 100 ng of Entranceposon (M1-Kanamycin), 1 μl of MuA transposase, and 4 μl of the manufacturer&#39;s 5×MuA reaction buffer in a total volume of 20 μl. The transposition reaction was allowed to proceed for 60 minutes at 30° C., and the MuA transposase was subsequently inhibited by incubation at 75° C. for 10 minutes.  
      Plasmid DNA was isolated and purified into a volume of 15 μl, 1 or 3 μl thereof was then electrotransformed into competent  E. coli  cells according to standard procedures, and transformants were spread out on LB-kanamycin plates (10 μg/ml) yielding 16,000 and 65,000 kanamycin resistent transformants, respectively. The procedure was repeated yielding a total number of about 1×10 6  transformants. Transposon-containing plasmid DNA was purified from overnight incubations of selected transformed  E. coli  cells in LB-ampicillin (100 μg/ml) and kanamycin (10 μg/ml) medium.  
      Isolation of genes with transposons. In order to isolate genes with transposons, 10 μg of the above purified plasmid was digesed with Pac I and Xba 1, which should result in four DNA fragments: the original vector, the gene fragment, plus vector- and gene fragments with inserted transposon. The amylase encoding gene fragment with the transposon inserted was isolated by agarose gel electrophoresis and cloned back into Pac I and Xba I digested pMiBg242. Approximately, 600,000 kanamycin-resistant transformants were obtained.  
      Introduction of DNA fragment with outside cutter sites. Not 1-digested DNA-fragments flanked with outside cutters were introduced into the library of maltogenic amylase genes (with transposons) in the two flanking Not I-sites of the inserted transposon: 10 μg of plasmid DNA of the amylase encoding gene (with transposons) was digested with Not I and the vector- and gene-fragments were isolated from the transposon fragment and ligated to the Not I restricted PCR-fragments. More than 600,000 kanamycin-resistant transformants were obtained.  
      Trimming flanking sites and excision of transposon by Bsq I restriction. The transposon fragment and parts of the flanking sequences were digested from the construct with Bsg I of two Bsg I sites at each end of the inserted transposon and the linearized vector was purified from an agarose gel. The position of one of the Bsg I sites was designed so that Bsg I restriction would remove all of the five duplicated base pairs plus two more base pairs (right site in  FIG. 9B ). The position of the other Bsg I site was designed so that Bsg I restriction would remove base pair 5 (left site in  FIG. 9 ). The sticky-ends were then PCR polished and the vector was religated so that a triplet of basepairs was deleted. The circularized vector was then transformed into  E. coli  yielding more than 1×10 6  transformants.  
      Sequence analysis. DNA-sequence analysis of six different resulting variants gave following DNA and amino acid deletions (‘D260*’ means residue D260 is deleted):  
      One Deleted Codon Triplets:  
                                                      Variant 1: D260*   Position:       259   260   261                   Amino acid sequence of wt:   N-term.       G   D   D       C-term.       Coding sequence of wt:       5′   GGA   GAT   GAC   3′       Coding sequence of variant 1:       5′   GGA   -   GAC   3′           N-term.       G   -   D       C-term.          
 
      Two Deleted Codon Triplets were Also Constructed:  
                                                      Variant 2: V129*, P130*   Position:       128   129   130   131               Amino acid sequence of wt:   N-term.       F   V   P   N   C-term.       Coding sequence of wt:       5′   TTT   GTG   CCC   AAT   3′       Coding sequence of variant 2:       5′   TT-   ---   --C   AAT   3′           N-term.       F   -   -   N   C-term.               Variant 3: N131*, H132*   Position:       130   131   132   133       Amino acid sequence of wt:   N-term.       P   N   H   S   C-term.       Coding sequence of wt:       5′   CCC   AAT   CAT   TCG   3′       Coding sequence of variant 3:       5′   CC-   ---   --T   TCG   3′           N-term.       P   -   -   S   C-term.                                                             Variant 4: S476T, V477*, A478*   Position:       475   476   477   478   479               Amino acid sequence of wt:   N-term.       G   S   V   A   S   C-term.       Coding sequence of wt:       5′   GGA   AGT   GTC   GCT   TCG   3′       Coding sequence of variant 4:       5′   GGA   A--   ---   -CT   TCG   3′           N-term.       G   T   -   -   S   C-term.                             Three deleted codon triplets were also constructed:                                                         Variant 5: V254*, G255*, E256   Position:       253   254   255   256   257               Amino acid sequence of wt:   N-term.       L   V   G   E   W   C-term.       Coding sequence of wt:       5′   CTG   GTG   GGG   GAA   TGG   3′       Coding sequence of variant 5:       5′   CTG   GTG   GGG   GAA   TGG   3′           N-term.       L   -   -   -   W   C-term.                                                                 Variant 6: H267Q, L268*, E269*, K270*.                                               Amino acid sequence of wt:   Position:       266   267   268   269   270   271       Coding sequence of wt:   N-term.       N   H   L   E   K   V   C-term.       Coding sequence of variant 6:       5′   AAT   CAT   CTG   GAA   AAG   GTC   3′           5′   AAT   CA-   ---   ---   --G   GTC   3′           N-term.       N   Q   -   -   -   V   C-term.          
 
      The invention described and claimed herein is not to be limited in scope by the specific aspects herein disclosed, since these aspects are intended as illustrations of several aspects of the invention. Any equivalent aspects are intended to be within the scope of this invention. Indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the appended claims. In the case of conflict, the present disclosure including definitions will control.  
      Various references are cited herein, the disclosures of which are incorporated by reference in their entireties.