CONSTRUCTING AND SCREENING A DNA LIBRARY OF INTEREST IN FILAMENTOUS FUNGAL CELLS

Abstract: A method of constructing and screening a library of polynucleotide sequences of interest in filamentous fungal cells by use of an episomal replicating AMA1-based plasmid vector, thus achieving a high frequency of transformation and a stable and standard uniformly high level of gene expression.

Description: CONSTRUCTING AND SCREENING A DNA LIBRARY OF INTEREST IN FILAMENTOUS FUNGAL CELLS Field of the Invention The present invention relates to methods of constructing and screening a library of polynucleotide sequences of interest in filamentous fungal cells. Background of the Invention Filamentous fungi have been widely used as host cells for the commercial production of polypeptides. However, when it is desirable to produce a variant of the polypeptide with speci- fied altered characteristics, e.g., thermostability, pH activity profile, specific activity, substrate specificity, K.,, V max etc., the construction and screening of a library of variant encoding sequences commonly requires the use of an intermediate host, e.g., bacterial cells or yeast, due to the low frequency of transformation and a variation in copy number among independently transformed filamentous fungal cells. Several methods for the construction of libraries of polynucleotide sequences of interest in yeast have been disclosed in which the libraries are screened in yeast prior to transformation of a production relevant host, such as, for example, filamentous fungi with the potential variant polynucleotide sequences of interest. Often however, a polynucleotide sequence identified by screening in yeast or bacteria cannot be expressed or is ex- pressed at low levels when transformed into production relevant filamentous fungal cells. This may be due any number of reasons, including differences in codon usage, regulation of mRNA levels, translocation apparatus, post-translational modification machinery (e.g., cysteine bridges, glycosylation and acy- lation patterns), etc. Secondly, whether a polynucleotide sequence of interest would be expressed in the production host at commercially useful levels is not necessarily predictable. For example, if the organism used in screening the library is, e.g., a bacterial or yeast cell and the production relevant host cell is a filamentous fungal cell, the protease profiles differ. Thus, a se- quence encoding one or more characteristics of interest which has been identified in yeast may be degraded by proteases expressed in the product relevant filamentous fungal host cell. Furthermore, to obtain optimized yields of the expressed product by altering the function of regulatory proteins or regula- tory sequences requires direct manipulation of the production host . A. Aleksenko and A.J. Clutterbuck (1997. Fungal Genetics and Biology 21:373-387) disclose the use of autonomous replicative vectors, or autonomously replicating sequences (ARS) , for gene cloning and expression studies. AA1 (autonomous maintenance in Aspergillus) is one of the plasmid replicator elements discussed. It consists of two inverted copies of a genomic repeat designated MATE1 (mobile Aspergillus transformation enhancer) separated by a 0.3 kb central spacer. AMA1 promotes plasmid replication without rearrangement, multimerization or chromosomal integration. Summary of the Invention It has been found that AMA1-based plasmids provide two advantages in gene cloning in filamentous fungi. The first is a high frequency of transformation which both increases the potential library size and can eliminate the need for library amplification in an intermediate host, e.g., E. coli , so that a recipient Aspergillus strain can be transformed directly with a ligation mixture. Secondly, by providing a stable and standard environment for gene expression, the properties of the transformants will be uniform. It is an objective of the present invention to provide improved methods for constructing and screening libraries of polynucleotide sequences of interest in filamentous fungal cells by use of an episomal replicating DNA vector to provide a high frequency of transformation and a uniformly high level of gene expression among independently transformed cells. By minimizing variation in copy number among independently transformed cells, a variant polypeptide of interest may be identified directly on the basis of expression of the characteristic (s) of interest . Accordingly, in a first aspect the present invention relates to a method of constructing and selecting or screening a library of polynucleotide sequences of interest in filamentous fungal cells wherein the method comprises: (a) transforming the fungal cells with a population of DNA vectors, wherein each vector comprises: (i) a polynucleotide sequence encoding a fungal selective marker and a fungal replication initiating sequence, wherein the marker and the replication initiating sequence do not vary within the population; and (ii) a polynucleotide sequence of interest wherein the population of DNA vectors contains more than one variant of the polynucleotide sequence; (b) cultivating the cells under selective pressure; (c) selecting or screening for one or more transformants expressing a desired characteristic; and (d) isolating the transformant (s) of interest. In other aspects, the invention relates to the use of a fungal replication initiating sequence to construct a library of polynucleotide sequences of interest and to screen or select a library of such polynucleotide sequences. Brief Description of the Figures Figure 1 shows a restriction map of the plasmid pENI1298, the construction of which is described in Example 1; Figure 2 a restriction map of the plasmid pENI1299, the construction of which is described in Example 1; Figure 3 a restriction map of plasmids pDM156.2, pJRoy47 and pDM222.A described in Example 5; and Figure 4 a restriction map of plasmid pJRoy44 described in Example 5. Detailed Description of the Invention In a first embodiment, the present invention relates to a method of constructing and selecting or screening a library of polynucleotide sequences of interest in filamentous fungal cells, wherein the method comprises: (a) transforming the fungal cells with a population of DNA vectors, wherein each vector comprises: (i) a polynucleotide sequence encoding a fungal selective marker and a fungal replication initiating sequence, wherein the marker and the replication initiating sequence do not vary within the population; and (ii) a polynucleotide sequence of interest, wherein the population of DNA vectors contains more than one variant of the polynucleotide sequence; (b) cultivating the cells under selective pressure; (c) selecting or screening for one or more transformants expressing a desired characteristic and (d) isolating the transformant (s) of interest. The term, "a library of polynucleotide sequences of interest" denotes a collection of polynucleotide sequences which en- code or have the same function or activity of interest. The library may be "a library of variants of a polynucleotide sequence of interest" which is defined herein as a collection of variants wherein the variants differ from a parent polynucleotide sequence (s) by comprising one or more modifications of said parent polynucleotide sequence. Conveniently, the polynucleotide sequences or variants are generated from at least one parent polynucleotide sequence of interest by mutagenesis, preferably random mutagenesis, resulting in a minimum number of 4 and preferably a minimum of 25 different polynucleotide se- quences in the collection. Various methods useful for mutagenizing a parent polynucleotide sequence are described in the sections further below entitled "Random mutagenesis" and "Localized random mutagenesis". The term "modification (s) " is intended to indicate a substitution, insertion and/or deletion of one or more nucleotides in the sequence as compared to that of the variant and may include a hybrid of two or more different parental polynucleotides. The polynucleotide sequences or variants may also result from naturally occurring allelic variation of a parent polynu- cleotide sequence. An allelic variant denotes any of two or more alternative forms of a gene occupying the same chromosomal locus. Allelic variation arises naturally through mutation, and may result in phenotypic polymorphism within populations. Gene mutations can be silent (i.e., no change in the encoded polypeptide) or may encode polypeptides having altered amino acid sequences. Furthermore, rather than being generated from one or more parent nucleotide sequences the polynucleotide sequences of the library may be derived from different sources and encode different, but highly related polypeptides having the same activity or function. In the present context, "highly related" is intended to indicate that the polypeptides have the same activity or function and in addition are encoded by a polynucleotide sequences having shared conserved regions as defined in the section further below entitled "DNA shuffling" . The term "derived from" is intended to indicate that the polynucleotide sequence is isolated from a specific source, such as a microorganism. The polynucleotide sequence may be one generated from a non-cultivable organism. The polynucleotide sequence (s) may be any polynucleotide sequence having or encoding a biological activity or function of interest and comprising a subsequence of at least six nucleotides to allow for pro- duction of a variant nucleotide sequence. The term "same activity or function" as used about the polynucleotide sequences contained in the library is intended to indicate that the polynucleotide sequences encode polypep- tides having the same or a similar biological activity or function (determined qualitatively) or the polynucleotide sequences themselves have the same (qualitative) activity or function. For instance, the polynucleotide sequences may have promoter activity, ie be capable of promoting transcription, or may en- code polypeptides with the same qualitative activity or function such as the same enzymatic activity or function, eg protease, lipase or amylase activity. The term "DNA vector" is a polynucleotide sequence, which has the ability to replicate autonomously and which is able to contain a sequence of interest . Thus a DNA vector contains a replication initiation sequence, such as an origin (e.g. ColEl origin, FI origin, M13 origin, 2μ origin (yeast) , oriP (EB virus) (pi) . The DNA vector may e.g. be a plasmid. The term "wherein the population of DNA vectors contains more than one variant of the polynucleotide sequence" is intended to indicate that the population of DNA vectors contain different polynucleotide sequences encoding or having a biological function or activity of interest. In a preferred embodiment the polynucleotide sequences of interest are polypeptide encoding sequences. The term "polypeptide" encompasses peptides, oligopeptides, and proteins and, therefore, is not limited to a specific length of the encoded product. The polypeptide may be native to the host cell or may be a heterologous polypeptide. The term "heterologous polypeptide" is defined as a polypeptide which is not native to the host cell. The polypeptide may also be a recombinant polypeptide which is a polypeptide native to a cell, which is encoded by a nucleic acid sequence which comprises one or more control sequences, foreign to the nucleic acid sequence, which are involved in the production of the polypeptide. The nucleic acid sequence encoding the polypeptide may have been manipulated in some manner as described infra . The polypeptide may be a wild-type polypeptide, or a variant thereof, subjected to the method of the present invention. The polypeptide may also be a hybrid polypeptide which contains a combination of partial or complete polypeptide sequences obtained from at least two different polypeptides where one or more of the polypeptides may be heterologous to the cell. Polypeptides further include naturally occurring allelic and engineered variations of the above mentioned polypeptides. In another preferred embodiment the polynucleotide sequence of interest is a control sequence normally associated with a polypeptide encoding sequence. Control sequences include all components which are operably linked to the nucleic acid sequence encoding the polypeptide sequence or otherwise involved in the production of the polypeptide. Such control sequences include, but are not limited to, a promoter, a signal sequence, a propeptide sequence, a transcription terminator, a leader, a promoter recognition sequence, an enhancer sequence and a poly- adenylation sequence as described herein. In a still further embodiment the polynucleotide sequence of interest is a combination of a polypeptide encoding sequence (or a part of such sequence) and a) a control sequence (or part of such control sequence) or b) two or more control sequences (or parts of such sequences) . Each of the control sequences may be native or foreign to the coding sequence. In a preferred embodiment, the polypeptide encoded by a polynucleotide sequence of interest is an antibody or portions thereof, an antigen, a clotting factor, an enzyme, a hormone or a hormone variant, a receptor or portions thereof, a regulatory protein, a structural protein, a reporter, or a transport protein. In a more preferred embodiment, the enzyme is an oxidoreduc- tase, transferase, hydrolase, lyase, isomerase, or ligase. In an even more preferred embodiment, the enzyme is an amin- opeptidase, amylase, carbohydrase, carboxypeptidase, catalase, cellulase, chitinase, cutinase, deoxyribonuclease, dextranase, esterase, alpha-galactosidase, beta-galactosidase, glucoamy- lase, alpha-glucosidase, beta-glucosidase, haloperoxidase, in- vertase, laccase, lipase, mannosidase, mutanase, oxidase, pectinolytic enzyme, peroxidase, phytase, polyphenoloxidase, proteolytic enzyme, ribonuclease, transglutaminase, or xyla- nase . A specific example of a lipase is a lipase derived from Thermomyces lanuginosa or a variant of said lipase, eg a vari- ant in which an N-terminal extension has been added to the mature lipase enzyme as disclosed in WO 97/04079. In another embodiment, the polypeptide is human insulin or an analog thereof, human growth hormone, erythropoietin, or in- sulinotropin . In a further preferred embodiment the control sequence is a promoter sequence, preferably a fungal promoter, such as a promoter derived from the gene encoding Aspergillus oryzae TAKA amylase, NA2-tpi (a hybrid of the promoters from the genes encoding A . niger neutral α-amylase and A . oryzae triose phos- phate isomerase) and Aspergillus niger or Aspergillus awamori glucoamylase . In another preferred embodiment the control sequence is a promoter recognition sequence such as the amyR recognition sequence (WO 98/01470) , the creA (WO 94/13820) and areA (WO 95/35385) . Examples of further control sequences of interest in connection with the present invention are listed further below in the section entitled "DNA vectors and control sequences" . Filamentous fungal selective marker The term "selective pressure" is defined herein as culturing a filamentous fungal cell, containing a DNA vector with a fungal selective marker gene and a polynucleotide sequence of in- terest, in the presence of an effective amount or the absence of an appropriate selective agent. The effective amount of the selective agent is defined herein as an amount sufficient for allowing the selection of cells containing the selection marker from cells which do not contain the selection marker. In a preferred embodiment, the fungal selective marker is selected from the group of genes which encode a product capable of providing resistance to biocide or viral toxicity, resistance to heavy metal toxicity, or prototrophy to auxotrophs . In a more preferred embodiment, the prototrophy is obtained from an enzyme selected from the group of metabolic pathways consisting of nucleotide synthesis, cofactor synthesis, amino acid synthesis, acetamide metabolism, proline metabolism, sulfate metabolism, and nitrate metabolism. In an even more preferred embodiment, the fungal selective marker is a gene selected from the group consisting of argB (ornithine carbamoyltransferase) , amdS (acetamidase) , Jbar (phosphinothricin acetyltransferase) , hemA (5-aminolevulinate synthase) , hemB (porphobilinogen synthase) , hygB (hygromycin phosphotransferase) , niaD (nitrate reductase) , prn (proline permease) , pyrG (orotidine-5' -phosphate decarboxylase) , pyroA, riboB, sC (sulfate adenyltransferase) , and trpC (anthranilate synthase) . The fungal cell is cultivated in a suitable medium and under suitable conditions for screening or selecting for transformants harbouring the variant polynucleotide sequence of interest having or encoding the desired characteristic. The cultiva- tion may be performed in accordance with methods well-known in the art for screening of polynucleotide sequence libraries. Replication initiating sequences As used herein, the term "fungal replication initiating sequence" is defined as a nucleic acid sequence which is capable of supporting autonomous replication of an extrachromosomal molecule, e.g., a DNA vector such as a plasmid, in a fungal host cell, normally without structural rearrangement of the DNA-vector or integration into the host cell genome. The replication initiating sequence may be of any origin as long as it is capable of mediating replication intiating activity in a fungal cell. For instance the replication initiating sequence may be a telomer of human origin which confer to the plasmid the ability to replicate in Aspergillus (Aleksenko and Ivanova, Mol. Gen. Genet. 260 (1998) 159-164). Preferably, the replication initiating sequence is obtained from a filamentous fungal cell, more preferably a strain of Aspergillus , Fusarium or Al - ternaria, and even more preferably, a strain of A . nidulans, A . oryzae, A . niger, F. oxysporum or Al ternaria al tenata . A fungal replication initiating sequence may be identified by methods well-known in the art. For instance, the se- quence may be identified among genomic fragments derived from the organism in question as a sequence capable of sustaining autonomous replication in yeast, (Ballance and Turner, Gene, 36 (1985) , 321-331) , an indication of a capability of autonomous replication in filamentous fungal cells. The replication initi- ating activity in fungi of a given sequence may also be determined by transforming fungi with contemplated plasmid replicators and selecting for colonies having an irregular morphology, indicating loss of a sectorial plasmid which in turn would lead to lack of growth on selective medium when selecting for a gene found on the plasmid (Gems et al, Gene, 98 (1991) 61-67) . AMA1 was isolated in this way. An alternative way to isolate a replication initiating sequence is to isolate natural occurring plasmids (eg as disclosed by Tsuge et al . , Genetics 146 (1997) 111-120 for Al ternaria aternata) . Examples of fungal replication initiating sequences include, but are not limited to, the ANSI and AMA1 sequences of Aspergillus nidulans, e.g., as described, respectively, by Cul- len, D., et al . (1987, Nucleic Acids Res. 15:9163-9175) and Gems, D., et al . (1991, Gene 98:61-67). The term "replication initiating activity" is used herein in its conventional meaning, ie to indicate that the sequence is capable of supporting autonomous replication of an ex- trachromosomal molecule, such as a plasmid or a DNA vector in a fungal cell. The term "without structural rearrangement of the plasmid" is used herein to mean that no part of the plasmid is deleted or inserted into another part of the plasmid, nor is any host genomic DNA inserted into the plasmid. Preferably, the replication initiating sequence to be used in the methods of the present invention is a nucleic acid sequence selected from the group consisting of: (a) a nucleotide sequence having at least 50% identity with the nucleic acid sequence of SEQ ID NO:l or SEQ ID NO: 2, and is capable of initiating replication in a fungal cell; (b) a nucleotide sequence capable of initiating replication which hybridises under low stringency conditions with (i) the nucleic acid sequence of SEQ ID NO:l or SEQ ID NO: 2, or (ii) the respective complementary strands, wherein the low stringency conditions are defined by prehybridisation and hybridisation at 42°C in 5x SSPE, 0.3% SDS, 200 mg/ml sheared and denatured salmon sperm DNA, and 25% formamide, and wash condi- tions are defined at 50°C for 30 minutes in 2x SSC, 0.2% SDS; and (c) a subsequence of (a) or (b) , wherein the subsequence is capable of initiating replication in a fungal cell. In a preferred embodiment, the nucleotide sequence has a degree of identity to the nucleic acid sequence shown in SEQ ID NO:l or SEQ ID NO : 2 of at least about 50%, more preferably about 60%, even more preferably about 70%, even more preferably about 80%, even more preferably about 90%, and most preferably about 97% identity (hereinafter "homologous polynucleotide"). The homologous polynucleotide also encompasses a subsequence of SEQ ID NO:l or SEQ ID NO : 2 which has replication initiating activity in fungal cells. For purposes of the present invention, the degree of identity may be suitably determined by means of computer programs known in the art, such as GAP provided in the GCG program package (Program Manual for the Wisconsin Package, Version 8, August 1994, Genetics Computer Group, 575 Science Drive, Madison, Wisconsin, USA 53711) (Needleman, S.B. and Wunsch, CD., (1970), Journal of Molecular Biology, 48, 443-45), using GAP with the following settings for polynucleotide sequence comparison: GAP creation penalty of 5.0 and GAP extension penalty of 0.3. Hybridisation indicates that by methods of standard Southern blotting procedures, the replication initiating sequence hybridises to an oligonucleotide probe derived from the nucleic acid sequence shown in SEQ ID NO:l or SEQ ID NO: 2, under low to high stringency conditions (i.e., prehybridisation and hybridisation at 42°C in 5x SSPE, 0.3% SDS, 200 μg/ml sheared and denatured salmon sperm DNA, and either 25, 35 or 50% formamide for low, medium and high stringencies, respectively). In order to identify a clone or DNA which is homologous with SEQ ID NO:l or SEQ ID NO : 2 , the hybridisation reaction is washed three times for 30 minutes each using 2X SSC, 0.2% SDS preferably at least 50°C, more preferably at least 55°C, more preferably at least 60°C, more preferably at least 65°C, even more preferably at least 70°C, and most preferably at least 75°C. The oligonucleotide probe may be the nucleic acid sequence of SEQ ID NO:l or SEQ ID NO : 2 , or a respective subsequence thereof. More - specifically, the oligonucleotide probe can be considerably shorter than the entire sequence, but should be at least 15, preferably at least 25, and more preferably at least 40 nucleotides in length. Both DNA and RNA probes can be used. The probes are typically labelled for detecting the corresponding gene (for example, with 32 P, 3 H, 35 S, biotin, or avidin) . For example, molecules to which a 32 P-, 3 H- or 35 S- labelled oligonucleotide probe hybridises may be detected by use of X-ray film. When isolating a replication initiating sequence for use in the present invention, a genomic DNA, cDNA or combinatorial chemical library prepared from such an organism as defined above expected to harbour the sequence is screened for DNA which hybridises with the oligonucleotide probe described above which has replication initating activity. Genomic or other DNA from such other organisms may be separated by agarose or polyacrylamide gel electrophoresis, or other separation techniques. DNA from the libraries or the separated DNA may be transferred to and immobilised on nitrocellulose or other suitable carrier material . A clone or DNA which is homologous to SEQ ID NO:l or SEQ ID NO : 2 may then be identified following standard Southern blotting procedures. The techniques used to isolate or clone a nucleic acid sequence having replication initiating activity are known in the art and include isolation from genomic DNA or cDNA. The cloning from such DNA can be effected, e . g. , by using methods based on polymerase chain reaction (PCR) 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) may be used. In preferred embodiment, the replication initiating sequence has the nucleic acid sequence set forth in SEQ ID NO:l or SEQ ID NO: 2, or a respective functional subsequence thereof. For instance, a functional subsequence of SEQ ID NO:l is a nucleic acid sequence encompassed by SEQ ID NO:l or SEQ ID NO 2 except that one or more nucleotides from the 5' and/or 3' end have been deleted. Preferably, a subsequence contains at least 100 nucleotides, more preferably at least 1000 nucleotides, and most preferably at least 2000 nucleotides. In a more preferred embodiment, a subsequence of SEQ ID NO:l contains at least the nucleic acid sequence shown in SEQ ID NO: 2. DNA vector and control sequences In the DNA vector comprising a polynucleotide sequence en- coding a fungal selective marker, a fungal replication initiating sequence and a polynucleotide sequence of interest the polynucleotide sequence may encode a polypeptide in which case it is operably linked to one or more control sequences which direct the expression of the coding sequence. Alternatively, the polynucleotide sequence is a control sequence in which case, depending on the control sequence in question, it is normally operably linked to a polypeptide encoding sequence in order to be able to assess the activity of the control sequence (and thus be able to select variants of the parent control se- quence having desired properties) . The procedures used to ligate the elements of the DNA vector are well known to one skilled in the art (see, e . g. , Sambrook et al . , 1989, supra) . The term "operably linked" is defined herein as a configura- tion in which a control sequence is appropriately placed at a position relative to the polypeptide encoding sequence such that the control sequence directs the expression of the polypeptide or is otherwise involved in the production of the polypeptide. In the following different control sequences are discussed in further detail . The control sequences are those which are operably linked to the polynucleotide sequences of interest (when the polynucleotide sequences of interest encodes a polypeptide) , and those which constitute the polynucleotide sequences of interest (when the polynucleotide sequence is a control sequence) . It will be understood that one and the same control sequence may be used either as a polynucleotide sequence of interest (when the library is a library of control sequences) or as a control sequence involved in the production of a polypeptide encoded by a polynucleotide sequence of interest (when the library is a library of polynucleotide se- quence of interest encoding a polypeptide) and the following disclosure is intended to cover both types of use of the control sequence . The control sequence may be an appropriate promoter sequence, a nucleic acid sequence which is recognized by a host cell for expression of a polypeptide encoding sequence. The promoter sequence contains transcriptional control sequences which mediate the expression of the polypeptide. The promoter may be any nucleic acid sequence which shows transcriptional activity in the host cell of choice including mutant, trun- cated, 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 a polypeptide encoding nucleotide sequence in a fila- mentous fungal host cell are promoters obtained from the genes encoding Aspergillus oryzae TAKA amylase, Rhizomucor miehei aspartic proteinase, Aspergillus niger neutral alpha-amylase, As- pergillus 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 oxysporum trypsin-like protease (U.S. Patent No. 4,288,627), and mutant, truncated, and hybrid promoters thereof. Particularly preferred promoters for use in filamentous fungal host cells are the TAKA amylase, NA2-tpi (a hybrid of the promoters from the genes encoding Aspergillus niger neu- tral alpha-amylase and Aspergillus oryzae triose phosphate isomerase), and glaA promoters. The control sequence may also be a suitable transcription terminator sequence, a sequence recognized by a filamentous fungal cell to terminate transcription. The terminator sequence is operably linked to the 3' terminus of the polypeptide encoding nucleotide sequence. Any terminator which is functional in the filamentous fungal cell may be used in the present invention. Preferred terminators are obtained from the genes encoding Aspergillus oryzae TAKA amylase, Aspergillus niger glucoamylase, Aspergillus nidulans anthranilate synthase, Aspergillus niger alpha-glucosidase, and Fusarium oxysporum trypsin-like protease. The control sequence may also be a suitable leader sequence, a nontranslated region of a mRNA which is important for translation by the filamentous fungal cell. The leader sequence is operably linked to the 5' terminus of the polypeptide encoding nucleotide sequence. Any leader sequence which is functional in the filamentous fungal cell may be used in the present invention. Preferred leaders are obtained from the genes encoding Aspergillus oryzae TAKA amylase and Aspergillus nidulans triose phosphate isomerase. The control sequence may also be a polyadenylation sequence, a sequence which is operably linked to the 3' terminus of the polypeptide encoding nucleotide sequence which, when transcribed, is recognized by a filamentous fungal cell as a signal to add polyadenosine residues to transcribed mRNA. Any polyadenylation sequence which is functional in the filamentous fungal cell may be used in the present invention. Preferred polyadenylation sequences are obtained from the genes encoding Aspergillus oryzae TAKA amylase, Aspergillus ni ger glucoamylase, Aspergillus nidulans anthranilate synthase, and Aspergillus niger alpha-glucosidase . The control sequence may also be a signal peptide coding region, which codes for an amino acid sequence linked to the amino terminus of the polypeptide which can direct the encoded polypeptide into the cell's secretory pathway. The 5' end of the polypeptide encoding 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 normally 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 obtain enhanced secretion of the polypeptide. The signal peptide coding region may be obtained from a glucoamylase or an amylase gene from an Aspergillus species, or a lipase or proteinase gene from a Rhizomucor species. However, any signal peptide coding region which directs the expressed polypeptide into the secretory pathway of a filamentous fungal cell may be used in the present invention. An effective signal peptide coding region is the signal pep- tide coding region obtained from the Aspergillus oryzae TAKA amylase gene, Aspergillus niger neutral amylase gene, Rhizomucor miehei aspartic proteinase gene, or Humicola lanuginosa cellulase gene. The control sequence may also be a propeptide coding region, which 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 Rhizomucor miehei aspartic proteinase gene, or the Myceliophthora thermo- phila laccase gene (WO 95/33836) . Where both signal peptide and propeptide regions are present at the amino terminus 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. The polypeptide encoding nucleotide sequence may also be operably linked to one or more nucleic acid sequences which encode one or more factors that are advantageous for directing the expression of the polypeptide, e . g. , a transcriptional activator (e.g., a trans-acting factor), a chaperone, and a proc- essing protease. Any factor that is functional in a filamentous fungal cell may be used in the present invention. The nucleic acids encoding one or more of these factors are not nee- essarily in tandem with the polypeptide encoding nucleotide sequence . An activator is a protein which activates transcription of a polypeptide encoding nucleotide sequence (Kudla et al . , 1990, EMBO Journal 9: 1355-1364; Jarai and Buxton, 1994, Current Genetics 26: 2238-244; Verdier, 1990, Yeast 6: 271-297). The nucleic acid sequence encoding an activator may be obtained from the genes encoding Saccharomyces cerevisiae heme activator protein 1 (hapl) , Saccharomyces cerevisiae galactose metabolizing protein 4 {gal4) , Aspergillus nidulans ammonia regulation protein ( areA) , and Aspergillus oryzae alpha-amylase activator {amyR) . For further examples, see Verdier, 1990, supra and MacKenzie et al . , 1993, Journal of General Microbiology 139: 2295-2307. A chaperone is a protein which assists another polypeptide to fold properly (Hartl et al . , 1994, TIBS 19: 20-25; Bergeron et al . , 1994, TIBS 19: 124-128; Demolder et al . , 1994, Journal of Biotechnology 32: 179-189; Craig, 1993, Science 260: 1902- 1903; Gething and Sambrook, 1992, Nature 355: 33-45; Puig and Gilbert, 1994, Journal of Biological Chemistry 269: 7764-7771; Wang and Tsou, 1993, The FASEB Journal 7: 1515-11157; Robinson et al . , 1994, Bio/Technology 1: 381-384; Jacobs et al . , 1993, Molecular Microbiology 8: 957-966). The nucleic acid sequence encoding a chaperone may be obtained from the genes encoding Aspergillus oryzae protein disulphide isomerase or Saccharomyces cerevisiae calnexin, Saccharomyces cerevisiae BiP/GRP78, and Saccharomyces cerevisiae Hsp70. For further examples, see Gething and Sambrook, 1992, supra, and Hartl et al . , 1994, supra . A processing protease is a protease that cleaves a propeptide to generate a mature biochemically active polypeptide (Enderlin and Ogrydziak, 1994, Yeast 10: 67-79; Fuller et al . , 1989, Proceedings of the National Academy of Sciences USA 86: 1434-1438; Julius et al . , 1984, Cell 37: 1075-1089; Julius et al . , 1983, Cell 32: 839-852; U.S. Patent No. 5,702,934). The nucleic acid sequence encoding a processing protease may be ob- tained from the genes encoding Saccharomyces cerevisiae dipep- tidylaminopeptidase, Saccharomyces cerevisiae Kex2, Yarrowia lipolytica dibasic processing endoprotease ( prrS) , and Fusarium oxysporum metalloprotease (p45 gene) . It may also be desirable to add regulatory sequences which allow the regulation of the expression of the polypeptide relative to the growth of the filamentous fungal 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. The TAKA alpha-amylase promoter, Aspergillus niger glucoamylase promoter, and the Aspergillus oryzae glucoamylase promoter may be used as regulatory sequences . Other examples of regulatory sequences are those which allow for gene amplification, e . g. , the metallothionein genes which are amplified with heavy met- als. In these cases, the polypeptide encoding nucleotide sequence would be operably linked with the regulatory sequence. The introduction of the DNA vector into a filamentous fungal cell may involve a process consisting of protoplast formation, transformation of the protoplasts, and regeneration of the cell wall in a manner known per se . Suitable procedures for transformation of Aspergillus host cells are described in EP 238 023 and Yelton et al . , 1984, Proceedings of the National Academy of Sciences USA 81: 1470-1474. A suitable method of transforming Fusarium species is described by Malardier et al . , 1989, Gene 78: 147-156 or in WO 96/00787. Fungal Cells The fungal cells to be transformed with the population of DNA vectors are filamentous fungal cells. "Filamentous fungi" include all filamentous forms of the subdivision Eumycota and Oomycota (as defined by Hawksworth et al . , 1995, supra) . The filamentous fungi are characterized by a mycelial wall composed of chitin, cellulose, glucan, chito- san, mannan, and other complex polysaccharides . Vegetative growth is by hyphal elongation and carbon catabolism is obliga- tely 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 a preferred embodiment, the filamentous fungal cell is a cell of a species of, but is not limited to, Acremonium, Aspergillus, Fusarium, Humicola, Mucor, Myceliophthora, Neurospora, Penicillium, Scy- talidium, Thielavia, Tolypocladium, and Trichoderma . The filamentous fungal cell to be used in the present invention is normally chosen on the basis of the polynucleotide sequence of interest. For instance, if the polynucleotide of interest is a control sequence intended for use in an Aspergillus cell the filamentous fungal cell is normally an Aspergillus cell. Examples of filamentous fungal cells of use in the present invention include an Aspergillus cell, an Acremonium cell, a Fusarium cell, a Humicola cell, a Mucor cell, a Myceliophthora cell, a Neurospora cell, a Penicillium cell, a Thielavia cell, a Tolypocladium cell, and a Trichoderma cell. More specifically, the filamentous fungal cell is an Aspergillus awamori , Aspergillus foetidus, Aspergillus japonicus, Aspergillus nidulans, Aspergillus niger, or Aspergillus oryzae cell; a Fusarium bactridioides, Fusarium cerealis, Fusarium crookwel - lense, Fusarium culmorum, Fusarium graminearum, Fusarium grami - num, Fusarium heterosporum, Fusarium negundi , Fusarium ox- ysporum, Fusarium reticulatum, Fusarium roseum, Fusarium sam- bucinum, Fusarium sarcochroum, Fusarium sporotricioides , Fusarium sulphur eum, Fusarium torulosum, Fusarium trichothe- cioides, Fusarium venenatum cell or a Fusarium venenatum cell (Nirenberg sp . nov; a Humicola insolens cell or a Humicola lanuginosa cell; a Mucor miehei cell; a Myceliophthora ther o- phila cell; a Neurospora crassa cell; a Penicillium purpuro- genum cell; a Thielavia terrestris cell; or a Trichoderma harzianum, Trichoderma koningii , Trichoderma long ibrachia turn, Trichoderma reesei , or Trichoderma viride cell. Selecting or screening the library for transformants of interest It will be understood that the method to be used for performing the selection or screening step c) of the method of the invention will depend on the polynucleotide sequence of interest in question. The term "selecting or screening" for one or more transformants expressing a desired characteristic" as used in step c) is intended to indicate that the screening or selecting is performed so as to identify transformants containing a modified polynucleotide sequence of interest (which has been generated on the basis of the polynucleotide sequences of interest during the cultivation step b) which has the desired activity or function and optionally further desired characteristics as exemplified below. Thus, if the polynucleotide sequence of interest encodes a polypeptide with a certain activity or function the selection will only allow the transformants expressing a polypeptide with the desired activity or function, to grow. Thus, if the polynucleotide sequence of interest encodes a polypeptide with a certain activity or function the screening will be performed to identify transformants expressing a polypeptide with the desired activity or function. For instance, if the polynucleotide sequence of interest encodes an enzyme, such as a lipase, the selection or screening step c) will be performed to identify transformants expressing lipase activity. If it is desired that the lipase to be identified as a specific characteristic, such as a high thermostability, the screening is to be performed under conditions (typically temperatures) at which upases with the desired high thermostability can be identified. Analogously, if the polynucleotide sequence of interest is a control sequence such as a promoter sequence the selection or screening step c) is performed under condition in which promoter activity can be assessed. Typically, in the library the promoter polynucleotide sequences of interest are operably linked to a second sequence to be transcribed (eg a polypeptide encoding sequence) so that the promoter activity can be assayed with reference to the transcription of said second sequence. Library construction in bacterial or yeast hosts The present invention also relates to methods of constructing and screening or selecting a library of polynucleotide sequences of interest in a filamentous fungal cell, comprising: (a) transforming a culture of bacterial or yeast cells with a population of DNA vectors, wherein each vector comprises (i) a polynucleotide sequence encoding a filamentous fungal selective marker, a filamentous fungal replication initiating sequence, a bacterial or yeast selective marker and a bacterial or yeast replication initiating sequence, respec- tively, none of which vary substantially within the population of DNA vectors, and (ii) a polynucleotide sequence of interest, wherein more than one variant of the polynucleotide sequence is contained within the population of DNA vectors; (b) cultivating the bacterial or yeast cells under selective pressure; (c) isolating the DNA vectors from the transformants of (b) ; (d) transforming filamentous fungal cells with the DNA vectors of (c) ; (e) cultivating the filamentous fungal cells of (d) under selec- tive pressure; (f) selecting or screening for one or more filamentous fungal transformants expressing a desired characteristic; and (g) isolating the filamentous fungal transformant (s) of interest . The advantage of using yeast or bacteria as intermediate hosts is that the transformation frequency is 100-1000 fold higher than for filamentous fungal cells such as Aspergillus, resulting in a larger library when first transforming the manipulated and optionally ligated DNA vectors into yeast or bac- teria, and then transforming the isolated supercoiled vectors into the filamentous fungal cell. In a preferred embodiment, the library of polynucleotide sequences of interest is prepared by random mutagenesis or naturally occurring allelic variants of at least one parent polynu- cleotide sequence having or encoding a biological function of interest as described above . In another preferred embodiment, the bacterial cell is a strain of E. coli . In another preferred embodiment, the yeast cell is a strain of Saccharomyces sp., in particular, strains of Saccharomyces cerevisiae or Saccharomyces kluyveri or Schizosaccharomyces sp. Further examples of suitable yeast cells are strains of Kluyveromyces , such as K. lactis, Hansenula, e . g. H. polymorpha, or Pichia, e . g. P. pastoris . According to the method above, the DNA vector contains a se- lective marker which permits easy selection of transformed bacterial or yeast cells. Examples of bacterial selective markers include, but are not limited to, markers which confer antibiotic resistance, such as ampicillin, kanamycin, erythromycin, chloramphenicol or tetracycline resistance. Furthermore, selection may be accomplished by co-transformation, e . g. , as described in WO 91/09129, where the selective marker is on a separate vector. Suitable markers for yeast host cells are ADE2 , HIS3 , LEU2 , LYS2, MET3, TRP1, and URA3. In a preferred embodiment, the bacterial or yeast selective marker provides for resistance to a biocide, wherein the bio- cide agent is selected from the group consisting of ampicillin, kanamycin, tetracyclin, chloramphenicol, neomycin, hygromycin and methotrexate . In another preferred embodiment, the bacterial or yeast se- lective marker is selected from the group of genes which encode a product which provides for resistance to biocide or viral toxicity, resistance to heavy metal toxicity, or prototrophy to auxotrophs . In another preferred embodiment, the prototrophy is obtained from an enzyme selected from the group of metabolic pathways consisting of nucleotide synthesis, cotactor synthesis, amino acid synthesis, acetamide metabolism, proline metabolism, sulfate metabolism, and nitrate metabolism. For autonomous replication, the vector may further comprise an origin of replication enabling the vector to replicate autonomously in the bacterial or yeast cell in question. Examples of bacterial origins of replication are the origins of rep- lication of plasmids pBR322, pUC19, pACYC177, and pACYC184 permitting replication in E. coli . 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. The transformation of the bacterial or yeast cell may, for instance, be effected by using competent cells, by electropora- tion, or by conjugation of bacterial cells (see, e . g. , J. Sam- brook, E.F. Fritsch, and T. Maniatis, 1989, Molecular Cloning, A Laboratory Manual , 2d edition, Cold Spring Harbor, New York) . The cultivation of these cells under selective pressure may be conducted in accordance with methods known in the art . The filamentous fungal selective marker, fungal replication initiating sequence and the polynucleotide sequence of interest is preferably as herein, eg in the sections entitled: "Filamentous fungal selective marker" and "Replication initiating sequences" . The selection or screening of transformants of interest may be performed as described above in the section entitled "Selection and screening of transformants" . Polynucleotide sequences of interest It will be understood that the present invention will be use- ful for screening or selecting libraries of any type of polynucleotide sequence of interest. More specifically, the polynucleotide sequences of interest to be used in the methods of the present invention may be generated from a parent polynucleotide sequence having or encoding a biological activity or func- tion of interest. For instance, the polynucleotide sequences of interest are generated from a gene encoding a polypeptide of interest by random mutagenesis of the gene. Alternatively, the polynucleotide sequence of interest is a control sequence as defined above, e.g., a promoter sequence. Also, the polynucleotide sequences of interest may be derived from different sources, such as different naturally occurring sources such as microorganisms, as described further above. The polynucleotide sequence of interest may be a combination of a polypeptide encoding sequence and a control sequence . In a preferred embodiment the modification of a parent polynucleotide sequence is done by use of a physical or chemi- cal mutagenizing agent, use of a doped oligonucleotide, DNA shuffling, or by subjecting the nucleic acid sequence to PCR generated mutagenesis, or use of any combination thereof. Alternatively, the polynucleotide sequences of interest may be obtained by subjecting the sequence to mutagenesis by misin- corporation of nucleotide bases by using an error-prone polymerase or a polymerase working under suboptimal conditions in order to promote formation of errors, i.e., error-prone PCR. Error-prone DNA synthesis may be carried out in vitro or in vivo, such as by use of various mutator strains. Random mutagenesis A general approach to generate variant polynucleotide sequences encoding modified proteins and enzymes has been based on random mutagenesis, for instance, as disclosed in US 4,894,331 and WO 93/01285. This approach may also be used in connection with the present invention. For instance, the random mutagenesis may be performed by use of a suitable physical or chemical mutagenizing agent, by use of a suitable oligonucleotide, or by subjecting the polynucleotide sequence to PCR gener- ated mutagenesis. Furthermore, the random mutagenesis may be performed by use of any combination of these mutagenizing agents. The mutagenizing agent may, e.g., be one which induces transitions, transversions, inversions, scrambling, deletions, and/or insertions . Examples of a physical or chemical mutagenizing agent suitable for the present purpose include ultraviolet (UV) ir- radiation, hydroxylamine, N-methyl-N' -nitro-N-nitrosoguanidine (MNNG) , O-methyl hydroxylamine, nitrous acid, ethyl methane sul- phonate (EMS) , sodium bisulphite, formic acid, and nucleotide analogues. When such agents are used, the mutagenesis is typically performed by incubating the polynucleotide sequence of in- terest in the presence of the mutagenizing agent of choice under suitable conditions for the mutagenesis to take place. When the mutagenesis is performed by the use of an oligonucleotide, the oligonucleotide may be doped or spiked with the three non-parent nucleotides during the synthesis of the oli- gonucleotide at the positions which are to be changed. The doping or spiking may be done so that codons for unwanted amino acids are avoided. The doped or spiked oligonucleotide can be incorporated into the polynucleotide of interest by any published technique, using e.g. PCR, LCR or any DNA polymerase and ligase as deemed appropriate . Preferably, the doping is carried out using "constant random doping", in which the percentage of wild-type and mutation in each position is predefined. Furthermore, the doping may be directed toward a preference for the introduction of certain nucleotides, and thereby a preference for the introduction of one or more specific amino acid residues. The doping may be made, e.g., so as to allow for the introduction of 90% wild type and 10% mutations in each position. An additional consideration in the choice of a doping scheme is based on genetic as well as protein-structural constraints. The doping scheme may be made by using the DOPE program (cf., Tomandl, D., et al., 1997. Journal of Computer-Aided Molecular Design 11:29-38; Jensen, LJ, Andersen, KV, Svendsen, A., and Kretzschmar, T., 1998. Nucleic Acids Research 26:697-702) which, inter alia, ensures that introduction of stop codons is avoided. When PCR-generated mutagenesis is used, either a chemically treated or non-treated parent polynucleotide sequence of interest is subjected to PCR under conditions that increase the mis- incorporation of nucleotides (Deshler, J.O., 1992. Genetic Analysis, Techniques and Applications 9:103-106; Leung, et al . , 1989. Technique 1:11-15. A mutator strain of E. coli (Fowler, et al . , 1974. Molec . Gen. Genet. 133:179-191), S. cereviseae or any other microbial organism may be used for the random mutagenesis of the parent polynucleotide sequence of interest by, e.g., transforming a plasmid containing the parent polynucleotide sequence into the mutator strain, growing the mutator strain with the plasmid and isolating the mutated plasmid from the mutator strain. The mutated plasmid may be subsequently transformed into the expression organism. The polynucleotide sequence to be mutagenised may be conven- iently present in a genomic or cDNA library prepared from an organism harbouring the sequence. Alternatively, the sequence may be present on a suitable vector such as a plasmid or a bacterio- phage, which as such may be incubated with or otherwise exposed to the mutagenising agent . The polynucleotide sequence to be mutagenised may be in isolated form. It will be understood that the polynucleotide sequence to be subjected to random mutagenesis is preferably a cDNA or a genomic DNA sequence. The mutated DNA sequence may further comprise a DNA sequence encoding functions permitting expression of the mutated DNA sequence. DNA shuffling Alternative methods for rapid preparation of variants of a polynucleotide sequence of interest in accordance with the present invention include methods of in vivo or in vi tro DNA shuffling, wherein DNA shuffling is defined as recombination, either in vivo or in vi tro, of nucleotide sequence fragment (s) between two or more homologous polynucleotides resulting in output polynucleotides (i.e., polynucleotides which have been subjected to a shuffling cycle) containing a number of exchanged nucleotide fragments when compared to the input polynucleotides (i.e., the polynucleotides subjected to shuffling) . Shuffling may be accomplished either in vi tro or in vivo by recombination within a cell by methods described in the art, examples of which are listed below. For instance, H. Weber and Weissmann, C. (1983. Nucleic Acids Research 11:5661-5669) describe a method for modifying genes by in vivo recombination between two homologous genes, in which re- combinants are identified and isolated using a resistance marker . Pompon, et al . , (1989, Gene 83:15-24) describe a method for shuffling gene domains of mammalian cytochrome P-450 by in vivo recombination of partially homologous sequences in Saccharomyces cerevisiae by transforming Saccharomyces cerevisiae with a linearized plasmid with filled-in ends, and a DNA fragment being partially homologous to the ends of said plasmid. In WO 97/07205 a method is described in which polypeptide variants are prepared by -shuffling different nucleotide sequences of homologous DNA sequences by in vivo recombination using plasmid DNA as template. US patent no. 5,093,257 (Genencor International, Inc.) discloses a method for producing hybrid polypeptides by in vivo recombination. In a preferred embodiment, the variant polynucleotide sequences of interest are obtained by in vivo recombination between two or more homologous nucleic acid sequences of interest, comprising: (a) identifying at least one conserved region between the polynucleotide sequences of interest; (b) generating fragments of each of the polynucleotide sequences of interest, wherein the fragments comprise the conserved re- gion(s) of (a); and (c) recombining the fragments of (b) by using the conserved region (s) as (a) homologous linking point (s). Preferably, the polynucleotide sequences of interest encode a polypeptide or a part thereof or are control sequences as defined above or any combination of both. The term "conserved region" denotes a subsequence, preferably of at least 10 bp, shared by two or more sequences in which there is a degree of identity between the subsequences of at least about 50%, more preferably at least about 60%, even more preferably at least about 70%, even more preferably at least about 80%, even more preferably at least about 90%, and most preferably at least about 97%. In order for the conserved region to be used as a "linking point" between two sequences, the degree of identity between the sequences within the conserved region (s), is sufficiently high to enable hybridisation, e.g., under conditions described supra, between the sequences, whereby the conserved region serves as the linking point . One method for shuffling of homologous DNA sequences in vi tro has been described by Stemmer (Stemmer, 1994. Proc. Natl. Acad. Sci. USA, 91:10747-10751; Stemmer, 1994. Nature 370:389-391; Crameri, A., et al . , 1997. Nature Biotechnology 15:436-438). The method relates to shuffling homologous DNA sequences by us- ing in vi tro PCR techniques. Positive recombinant genes containing shuffled DNA sequences are selected from a DNA library based on the improved function of the expressed proteins. The above method is also described in WO 95/22625 in relation to a method for shuffling homologous DNA sequences. An important step in the method is to cleave the homologous template double-stranded polynucleotide into random fragments of a desired size followed by homologously reassembling the fragments into full-length genes. WO 98/41653 discloses a method of DNA shuffling in which a library of recombined homologous polynucleotides is constructed from a number of different input DNA templates and primers by induced template shifts during in vi tro DNA synthesis. Localized random mutagenesis The random mutagenesis may be advantageously localised to a part of a parent polynucleotide sequence in question. The sequence to be modified may be, for example, the coding region of a gene, or a part thereof, essential for activity of the gene product, or a control sequence or part thereof as defined above. A preferred example of such control sequence is a promoter sequence, or a functional part thereof, i.e., a part which is sufficient for affecting expression of the nucleic acid sequence. Localised random mutagenesis may, e.g., be advantageous when certain regions of the polynucleotide sequence of interest have been identified to be of particular importance. For instance, when the polynucleotide sequence of interest encodes a polypeptide the important region may be one essential for a given prop- erty of the polypeptide, the modification of which region is expected to result in a variant in which this property has been improved. Such regions may normally be identified when the ter- tiary structure of the parent polypeptide has been elucidated and related to its function. Analogously, when the polynucleotide sequence of interest is a control sequence, such as a promoter, the region of interest may be one expected to be essen- tial to or involved in promoter activity. The localised, or region-specific, random mutagenesis is conveniently performed by use of PCR generated mutagenesis techniques as described above or any other suitable technique known in the art. Alternatively, the nucleotide sequence encoding the part of the polynucleotide sequence to be modified may be isolated, e.g., by insertion into a suitable vector, and said part may be subsequently subjected to mutagenesis by use of any of the mutagenesis methods discussed above. se of a fungal replication initiating sequence In a further aspect the invention relates to the use of a fungal replication initiating sequence as defined herein for constructing and selecting or screening a library of polynucleotide sequences of interest in filamentous fungal cells. Preferably, the library is constructed by the method according to the first or second aspect of the invention. In a preferred embodiment the fungal replication initiating sequence is selected from the group consisting of: (a) a replication initiating sequence having at least 50% identity with the nucleic acid sequence of SEQ ID NO:l or SEQ ID NO: 2 and is capable of initiating replication; (b) a replication initiating sequence which hybridises under low stringency conditions with (i) the nucleic acid sequence of SEQ ID NO : 1 or SEQ ID NO: 2, or (ii) the re- spective complementary strands, wherein the low stringency conditions are defined by prehybridization and hybridization at 42°C in 5x SSPE, 0.3% SDS, 200 mg/ml sheared and denatured salmon sperm DNA, and 25% forma- mide, and wash conditions are defined at 50°C for 30 minutes in 2x SSC, 0.2% SDS; and (c) a subsequence of (a) or (b) , wherein the subse- quence has replication initiating activity. Preferably, the replication initiating sequence is obtained from a filamentous fungal cell, in particular from a strain of Aspergillus, such as A. nidulans . In the most preferred embodiment the replication initiating sequence has the nucleic acid sequence set forth in SEQ ID NO : 1 or SEQ ID NO: 2, or is a respective functional subsequence thereof. The replication inti- ating sequence to be used in accordance with this aspect may be as described in the section above entitled "Replication initiating sequences" . Library of polynucleotide sequences of interest In a further aspect the invention relates to a library of polynucleotide sequences of interest which library comprises filamentous fungal cells transformed with a population of DNA vectors, wherein each vector comprises: (i) a gene encoding a fungal selection marker and a fungal replication initiating sequence wherein the marker and the replication initiating sequence do not vary within the population; and (ii) a polynucleotide sequence of interest wherein the population of DNA vectors contains more than one variant of the polynucleotide sequence. In a preferred embodiment the vector further comprises a nucleic acid sequence encoding a bacterial or yeast selective marker and a bacterial or yeast replication initiating sequence. Preferably, the library is prepared by the method according to the first or second aspect of the invention. Pref- erably, the elements of the library, such as the DNA vector and its mentioned components are as described herein. The present invention is further described by the following examples which should not be construed as limiting the scope of the invention. Examples Materials Chemicals used as buffers and substrates were commercial products of at least reagent grade. Plasmids pMT1505: constructed as described below in Example 1 pHelpl: contains the pyrG gene from A . oryzae as a selective marker and the AMA1 sequences which enable autonomous replication in A . nidulans as described by Gems, D., et al . (1991. Gene 98: 61-67) pToC68: as described in EP 0 531 372 (Novo Nordisk A/S) pAHL: as described in WO 92/05249, containing a lipase encoding sequence pS02 : as described in WO 96/29391, containing the Aspergil lus oryzae pyrG gene pENI1127: constructed as described below in Example 1 pENI1245: constructed as described below in Example 1 pENI1246: constructed as described below in Example 1 pENI1298: constructed as described below in Example 1 pENI1299: constructed as described below in Example 1 Strains JaL250: a derivative of Aspergillus oryzae A1560 in which the pyrG gene has been inactivated, as described in WO 98/01470 DH5a: an E. coli host cell purchased from GIBCO BRL (Life Technologies, Inc., Rockville MD) DLM15 : a derivative of Fusarium venenatum, as described in Example 5 below Example 1: Construction of pENI1298 and pENI1299 pMT1466 was constructed by inserting an Sphl/Narl fragment from pHelpl into pToC68. pMT1489 was constructed by digesting pMT1466 with Sphl and Stul, then religating. pMT1500 was con- structed by digesting pMT1489 with Aatll and Narl and ligating a linker. pMT1504 was constructed by digesting pMT1500 with Nhel and religating. pMT1505 was constructed by inserting a 2.7 kb Xbal fragment containing the amdS encoding gene from A . nidulans genomic DNA (Corrick, CM., et al . 1987, Gene 53:63- 71) into pMT1504 which had been cut with Nhel. pENI1127 was constructed from pMT1505 which had been digested with Sail in order to remove one of the AMAl repeats and the central spacer region. pENI1127 and pMT1505 were each cut with Hindlll, resulting in fragments of 2408 bp and 5253 bp, respectively, which were purified from a gel of 1% agarose and cloned into the Hindi11 site in the vector pAHL which contains a lipase encoding sequence. The resulting plasmids were called pENI1245 and pENI1246, respectively. A 3527 bp Stul/Bbul fragment containing the pyrG gene was excised from pS02 and inserted into a Stul/Bbul site in both pENI1245 and pENI1246. The resulting plasmids were called pENI1298 and pENI1299, respectively. The restriction map for pENI1298 is shown in Figure 1, and for pENI1299, in Figure 2. Example 2: Expression Levels of Lipase in Independently Grown Aspergillus oryzae Transformants JaL250 was transformed with pENI1298 and pENI1299 using standard procedures, cf., as described in WO 98/01470. The cells were then cultured on Cove plates at 37°C Transformants appeared after three days incubation at a transformation frequency of 10 4 - 10 5 /μg DNA, an increase of 100- to 10,000-fold over the transformation frequency in the absence of an AMAl sequence . Thirty independent transformants from the pENI1298 and pENI1299 transformations were isolated on Cove plates and at the same time inoculated into a 96-well microtiter dish containing 200 μl minimal media of l*vogel, 2% maltose (e.g., Methods in Enzymology. Vol. 17 p. 84) in each well. After three days of incubation at 34°C, media from the cul- tures in the microtiter dish were assayed for lipase activity. A 10 μl aliquot of media from each well was added to a microtiter well containing 200 μl of a lipase substrate of 0.018% p-nitrophenylbutyrate, 0.1% Triton X-100, 10 mM CaCl 2 , 50 mM Tris pH 7.5. Activity was assayed spectrophotometrically at 15-second intervals over a five minute period, using a kinetic microplate reader (Molecular Device Corp., Sunnyvale CA) , using a standard enzymology protocol (e.g., Enzyme Kinetics, Paul C.Engel, ed. , 1981, Chapman and Hall Ltd.) Briefly, product formation is measured during the initial rate of substrate turnover and is defined as the slope of the curve calculated from the absorbance at 405 nm every 15 seconds for 5 minutes. The arbitrary lipase activity units were normalized against the transformant showing the highest lipase activity. For each group of thirty transformants an average value and the standard deviation were calculated. At the same time, the transformants which had been cultured on Cove plates for three days at 37°C were reisolated onto a second Cove plate and reinoculated into a microtiter well as before. The procedure of assay, reisolation and reinoculation was repeated once more after an additional three days of culture . The results, summarized below in Table 1, show the amount of lipase produced, relative to the amount produced by the pENI1298 transformants six days after transformation. As indicated by the low values for the standard deviation, there is little variation in the lipase expression level among the 30 independently grown transformants. Usually, when doing transformation of filamentous fungi, a much larger relative standard deviation is seen (70%-100%) , due to both random integration into the genome of the vector and differences in numbers of vectors getting integrated. There can be 100-1000 fold differ- ence in expression levels between the worst and the best producing fungal transformant . Table 1. The average expression levels of lipase from 30 independently grown transformants relative to pENI1298 six days after transformation and the standard deviation for each. Example 3 : Testing for rearrangment of the plasmid Transformants of Jal250 containing either pENI1298 or pENI1299 were grown in YPD medium overnight. DNA was prepared from each of the transformants using a QIAprep ® Miniprep Kit (QIAGEN, Venlo, The Netherlands) in which the procedure provided by the manufacturer had been modified. Briefly, each strain was grown in 5 ml YPD for three days. The mycelia were collected by filtration and washed with 200 ml of water, then transferred to a 2 ml microfuge tube and lyophi- lized by centrifugation under vacuum for three hours at 60°C The dried mycelia was then ground and resuspended in one ml of lysis buffer (100 mM EDTA, 10 mM Tris pH8.0, 1 % tritonX-100, 500 mM guanidine-HCl, 200 mM NaCl) , followed by thorough mixing. Twenty mg RNAse was added to each tube which was then in- cubated at 37°C for 10 min. One hundred μg proteinase K was added, and the reaction was incubated for 30 min. at 50°C Each tube was then centrifuged for 15 min at top speed in a standard bench top microfuge. The supernatant was applied onto a QIAprep ® spin column, then centrifuged and filtrate discarded. The column was next washed in 0.5 ml PB provided in the kit, and centrifuged again for one minute. After the filtrate was discarded, the column was washed in 0.75 ml PE provided in the kit, then centrifuged once more for one minute. The column was allowed to air dry, and the DNA was eluted by addition of 100 μl TE buffer followed by a final one min spin. Transformants were obtained when the DNA was transformed into E. coli DH5a, confirming that the plasmids remained episo- mal in A . oryzae . Plasmid DNA was purified from the bacterial cells using QIAprep ® Miniprep Kit (QIAGEN) , according to the manufacturer's instructions. The purified plasmid DNA was then digested with Seal, and the restriction pattern was analyzed by standard agarose gel fractionation techniques. When compared to the restriction pattern of the original plasmid, the results showed no rearrangement in five pENI1299 JaL250 transformants and only one in eight pENI1298 JaL250 transformants, indicating the rarity of plasmid rearrangement events . Example 4 : Screening a Library In order to identify variant polypeptides with improved functional characteristics expressed at a level comparable to the parental polypeptide a library of variant polynucleotide sequences was constructed and screened in Aspergillus oryzae . Polymerase chain reactions, using pENI1298 as the template and 2 pmol/ml of each primer: oligo 19670 as one primer, and as a second primer, doped oligo 23701, 23702 or 23703, as described below, were run under the following conditions: 94°C, 5 min.; 30 cycles of (94°C, 30 sec; 50°C, 30 sec; 72°C, 1 min.), and 72°C, 5 min. A commercial Taq polymerase, AmpliTaq, was used as recommended by the supplier (Perkin-Elmer Corp., Branchburg NJ, USA) . 19670 SEQ ID NO: 3. 23701 SEQ ID NO: 4 23702 SEQ ID NO: 5 23703 SEQ ID NO: 6 The resulting products were purified using microfuge spin columns S300 (Pharmacia-LKB, Uppsala SE) . The purified products, including pENI1298 DNA, were then subjected to a second round of PCR amplification under the following conditions: 94°C, 5min.; 30 cycles of (94°C, 1 min.; 50°C, 10 min.; 72°C, 2 min.; and 72°C, 7 min., using the following primer: 19671: SEQ ID NO: 7. The products were next subjected to Dpnl digest to remove template DNA, and then incubated at 94°C for 30 min. to inactivate the Dpnl enzyme . JaL250 was transformed using 2 μg of pENI1298, which had been digested with Ball and SgrAl to remove most of the lipase encoding sequence, and 5 μg of one of the PCR fragments originating from the 23701, 23702 or 23703 doped-oligo reactions. The vector and the PCR fragment were allowed to recombine in vivo as described in WO 97/07205. JaL250 cells were also transformed with the digested pENI1298 or each of the PCR prod- ucts alone. One transformant was obtained when the cells were transformed with the vector alone, and no transformants resulted from transformation with the PCR fragment alone. 15, 12 and 26 transformants were inoculated from the 23701, 23702 and 23703 libraries, respectively, into microtiter plates containing 200 μl of l*vogel, 2% glucose media and incubated for 72 hours. Jal250 transformants of pENI1298 were also inoculated as a control . The cultures were then streaked onto Cove plates. Lipase activity in the microtiter cultures was assayed as described above in Example 2 and in a detergent con- taining assay in which 10 μl of the microtiter culture was added to 200 μl of a lipase substrate prepared using a commercial laundry detergent in microtiter wells. Activity was meas- ured spectrophotometrically at 405 nm and calculated as described above in Example 2. Each of the transformants which showed activity in the detergent containing assay was reisolated on a Cove plate. From each of the Cove plates that had been incubated for 72 hours at 37°C, two colonies were inoculated into a microtiter dish as described above, along with ten pENI1298 transformants, then cultured for 72 hours at 34°C. All the clones exhibited activity in the detergent assay, whereas the pENI1298 transformants did not. Furthermore, the transformants grown as independent duplicate cultures showed relatively similar levels of activity. The results are summarized below in Table 2. Table 2. Lipase activity on pnp-butyrate and a commercial laundry detergent . Because clone 23701.1 showed excellent expression as well as performance in both the pnp-butyrate and detergent assays, DNA was isolated in order to determine the deduced N-terminal sequence of the polypeptide, which was determined to be SPPRRPP . The naturally occurring N-terminal sequence is SPIRRE, which encodes a propeptide that is cleaved off by a kex2-like protease. DNA from some of the other transformants was sequenced, giving rise to the following deduced N-terminal sequences: 23701.2 (SPPRRRR) , 23702.1 (SPPRPRRR) , 23702.7 (SPPRPRRP) , 23703.1 (SPPRRRRRP) , and 23703.4 (SPPRRPRRR) . Thus, a variant polypeptide with improved functional characteristics which can be expressed at a level comparable to the parental polypeptide was identified in a production host. The invention described and claimed herein is not to be limited in scope by the specific embodiments herein disclosed, since these embodiments are intended as illustrations of several aspects of the invention. Any equivalent embodiments 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 modi- fications are also intended to fall within the scope of the appended claims . All publications mentioned herein are incorporated herein by reference for the purpose of describing and disclosing the par- ticular information for which the publication was cited. The publications discussed above are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the inventors are not entitled to antedate such disclosure by virtue of prior invention. It is to be understood that this invention is not limited to the particular methods and compositions described as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting since the scope of the present invention will be limited only by the appended claims . Unless defined otherwise all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any materials or methods similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described. Example 5: Construction of Fusarium venenatum strain DLM15 Strains Fusarium strain A3/5, now reclassified as Fusarium venenatum (Yoder and Christianson, 1998, Fungal Genetics and Biology 23: 62-80; O'Donnell et al . , 1998, Fungal Genetics and Biology 23: 57-67), was obtained from Dr. Anthony Trinci, University of Manchester, Manchester, England, or from the American Type Culture Collection, Manassas, VA, as Fusarium strain ATCC 20334. A morphological mutant of Fusarium venenatum A3/5 designated CCl-3 (Wiebe et al . , 1992, Mycological Research 96: 555-562; Wiebe et al . , 1991, Mycological Research 95: 1284-1288; Wiebe et al . , 1991, Mycological Research 96: 555-562) is a highly branched, colonial variant . The strain used in this experiment MLY3 was derived from the Fusarium venenatum A3/5 morphological mutant CCl-3. It has identical complementation characteristics to CC1- 3. Media and Solutions Minimal medium was composed per liter of 6 g of NaN0 3 , 0.52 g of KCl, 1.52 g of KH 2 P0 4 , 1 ml of COVE trace metals solution, 1 g of glucose, 500 mg of MgS0 4 -7H 2 0, 342.3 g of sucrose, and 20 g of Noble agar at pH 6.5. RA sporulation medium was composed per liter of 50 g of succinic acid, 12.1 g of NaN0 3 , 1 g of glucose, 20 ml of 50X Vogels, and 0.5 ml of a 10 mg/ml NaMo0 4 stock solution, pH to 6.0. YEPG medium was composed per liter of 10 g of yeast extract, 20 g of peptone, and 20 g of glucose. STC was composed of 0.8 M sorbitol, 25 mM Tris pH 8 , 25 mM CaCl 2 . SPTC was composed of 40% PEG 4000, 0.8 M sorbitol, 25 mM Tris pH 8, 25 mM CaCl 2 . COVE trace metals solution was composed per liter of 0.04 g of NaB 4 O 7 -10H 2 O, 0.4 g of CuS0 4 -5H 2 0, 1.2 g of FeS0 4 -7H 2 0, 0.7 g of MnS0 4 -H 2 0, 0.8 g of Na 2 Mo0 2 • 2H 2 0 , and 10 g of ZnS0 4 -7H 2 0. 50X COVE salts solution was composed per liter of 26 g of KCl, 26 g of MgS0 4 -7H 2 0, 76 g of KH 2 P0 4 , and 50 ml of COVE trace metals. COVE top agarose was composed per liter of 20 ml of 50X COVE salts, 0.8 M sucrose, 1.5 mM cesium chloride, 1.0 mM acetamide, and 10 g of low melt agarose, pH adjusted to 6.0. COVE medium was composed per liter of 342.3 g of sucrose, 20 ml of 50X COVE salt solution, 10 ml of 1 M acetamide, 10 ml of 1.5 M CsCl 2 , and 25 g of Noble agar. 50X Vogels medium was composed per liter of 150 g of sodium citrate, 250 g of KH 2 P0 4 , 10 g of MgS0 4 -7H 2 0, 10 g of CaCl 2 -2H 2 0, 2.5 ml of biotin stock solution, and 5.0 ml of AMG trace metals solution. Vogel' s/acetamide agar was composed per liter of 30 g of sucrose, IX Vogel's salts, 10 mM acetamide, 15 mM CsCl, and 25 g of Noble agar. Fluoroacetamide medium was composed per liter of 12 g of sodium acetate, 2 g of sodium chloride, 0.5 g of MgS0 4 , 3 g of KH 2 P0 4 , 0.3 g of urea, 2 g of fluoroacetamide, 1 ml of 50X Vogels salts, and 15 g of Agarose I (Amresco; Solon, Ohio), pH 6.1. Deletion of the pyrG gene in Fusarium venenatum MLY3 An unmarked pyrG deletion in the genome of the Fusarium venenatum expression host MLY3 was generated by initially deleting the pyrG gene and replacing it with the Aspergillus nidulans amdS gene flanked by repeated DNA sequences isolated from the upstream region of the Aspergillus oryzae pyrG gene. Isolates cured of the amdS gene were then selected by growth on plates containing fluoroacetamide. Plasmid pDM156.2 was constructed by inserting a 3.9 kb genomic FcoRI pyrG fragment cloned from Fusarium venenatum strain ATCC 20334 into the FcoRI site of pUCllδ . The pyrG fragment contained the entire coding region plus 1.3 kb of the promoter and 1.5 kb of the terminator (Figure 3) . The native pyrG gene of Fusarium venenatum MLY3 was replaced via homologous recombination by a 4.4 kb fragment from which the entire open reading frame of pyrG was deleted and replaced with the amdS gene flanked by 230 bp repeated sequences as shown in Figure 3. pJRoy47 was constructed first by constructing pJRoy43, which is composed of pNEB193 (New England Biolabs) where the Pad and Xfoal sites were replaced with a Swal site. To accomplish this, a linker containing a BamHI site on the 5' end, a Swal site in the middle, and a Sail site on the 3' end was created by annealing the following two oligos together: primer 1 : SEQ ID NO : 8. primer 2: SEQ ID NO: 9. The resulting linker was ligated into pNEB193, which had been digested with BamHI and Sail , to create pJRoy43. Next, a 230 bp region upstream of the Aspergillus oryzae pyrG gene was chosen as a repeat sequence in Fusarium venenatum. This region was amplified as two different products from the Aspergillus oryzae pyrG plasmid pJaL335 (WO 98/12300) using the following two primer pairs, primers 3 and 4 and primers 5 and 6 : Primer 3 SEQ ID NO: 10. Primer 4 SEQ ID NO: 11. Primer 5 SEQ ID NO: 12. Primer 6 SEQ ID NO: 13. The amplification reactions (50 μl) were prepared using approximately 40-50 ng of genomic DNA, prepared using the DNeasy Plant Mini Kit, as the template. Each reaction contained the following components: 40-50 ng of genomic DNA, 50 pmol each of the primers 3 and 4 or primers 5 and 6, 200 μM each of dATP, dCTP, dGTP, and dTTP, IX Tag DNA polymerase buffer, and 2.5 Units of Tag DNA polymerase. The reactions were incubated in a Perkin-Elmer Model 480 Thermal Cycler programmed as follows: Cycle 1 at 94°C for 2.5 minutes and 72°C for 2.5 minutes; cycles 2-26 each at 94°C for 45 seconds, 50°C for 45 seconds, and 72°C for 2 minutes; and cycle 27 at 94°C for 45 seconds, 50°C for 45 seconds, and 72°C for 10 minutes . The reaction products were isolated on a 1% agarose gel where approximately 230 bp fragments were isolated. The first PCR product (approximately 230 bp fragment) contained a Swal site at the 5' end and an FcoRV site at the 3' end while the second PCR product (approximately 230 bp fragment) contained an FcoRV site at the 5' end and a Pmel site at the 3' end. The two purified repeat fragments were first digested with FcoRV and ligated together. After purification (phenol-chloroform etraction and ethanol precipitation) , the ligation product was then digested with P el and Swal to produce approximately a 500 bp fragment which was purified by agarose gel electrophoresis and agarase treatment . The resulting fragment was cloned into Pmel and Swal digested pJRoy43 to create pJRoy44 (Figure 4) . This vector contains the two 230 bp repeats separated by an FcoRV site and flanked by Swal and Pmel sites. An FcoRI fragment containing the amdS gene and regulatory region was isolated from a subclone of p3SR2 (Kelly and Hynes, 1985, F BO Journal 4: 475-479), made blunt with the Klenow fragment, and ligated into FcoRV digested pJRoy44 to create pJRoy47 (figure 3) . The amdS gene flanked by the 230 bp repeated sequences was obtained from pJRoy47 as a Swal/Pmel fragment and inserted into FcoRV/StuI digested pDM156.2 to create pDM222.A (Figure 3). pDM222.A was digested with EcoRI and the 4.4 kb FcoRI fragment containing the pyrG deletion cassette was gel purified using Qiaquick Gel Extraction Kit (Qiagen, Chatsworth, CA) prior to transformation. Spores of Fusarium venenatum MLY3 were generated by inoculating a flask containing 500 ml of RA sporulation medium with three 1cm 2 mycelia plugs from a minimal medium plate and incubating at 28°C, 150 rpm for 2 to 3 days. Spores were harvested through Miracloth (Calbiochem, San Diego, CA) and centrifuged 20 minutes at 7000 rpm in a Sorvall RC-5B centrifuge (E. I. DuPont De Nemours and Co., Wilmington, DE) . Pelleted spores were washed twice with sterile distilled water, resuspended in a small volume of water, and then counted using a hemocytometer . Protoplasts were prepared by inoculating 100 ml of YEPG medium with 4 X 10 7 spores of Fusarium venenatum MLY3 and incubating for 16 hours at 24°C and 150 rpm. The culture was centrifuged for 7 minutes at 3500 rpm in a Sorvall RT 6000D (E. I. DuPont De Nemours and Co., Wilmington, DE) . Pellets were washed twice with 30 ml of 1 M MgS0 4 and resuspended in 15 ml of 5 mg/ml of NOVOZYME 234™ (batch PPM 4356, Novo Nordisk A/S, Bagsvasrd, Denmark) in 1 M MgS0 4 . Cultures were incubated at 24°C and 150 rpm until protoplasts formed. A volume of 35 ml of 2 M sorbitol was added to the protoplast digest and the mixture was centrifuged at 2500 rpm for 10 minutes. The pellet was resuspended, washed twice with STC, and centrifuged at 2000 rpm for 10 minutes to pellet the protoplasts. Protoplasts were counted with a hemocytometer and resuspended in an 8:2:0.1 solution of STCSPTCDMSO to a final concentration of 1.25 X 10 7 protoplasts/ml. The protoplasts were stored at -80°C, after controlled-rate freezing in a Nalgene Cryo 1°C Freezing Container (VWR Scientific, Inc., San Francisco, CA) . Frozen protoplasts of Fusarium venenatum MLY3 were thawed on ice. One μg of the 4.4 kb FcoRI fragment from pDM222.A described above and 5 μl of heparin (5 mg per ml of STC) were added to a 50 ml sterile polypropylene tube. One hundred μl of protoplasts was added, mixed gently, and incubated on ice for 30 minutes. One ml of SPTC was added and incubated 20 minutes at room temperature. After the addition of 25 ml of 40°C COVE top agarose plus 10 mM uridine, the mixture was poured onto a 150 mm diameter plate containing COVE agar to select for integration of the amdS gene. The native 3.9 kb FcoRI fragment of the pyrG gene of Fusarium venenatum MLY3 was replaced via homologous recombination by the 4.4 kb fragment containing pyrG flanking sequence and the amdS gene flanked by 230 bp repeated sequences. This resulted in the deletion of the entire pyrG coding region plus 0.78 kb of 5' untranslated and 0.8 kb of the 3' untranslated regions. Replacement of the native pyrG gene with the deletion cassette was confirmed by Southern hybridization. Southern hybridizations were conducted using the Amersham (Arlington, IL) Vistra and Rapid Hyb protocols or the Boehringer Mannheim DIG System protocol provided by the manufacturer. Fungal genomic DNA of putative deletants was prepared using the DNeasy Plant Mini Kit (Qiagen Chatsworth, CA) and digested with FcoRI . Southern blots were made using Magnagraph membrane (Micron Separations, Inc; Westborough, MA) and standard molecular biology techniques . Membranes were developed following the manufacturer's instructions and scanned using a STORM 860 (Molecular Dynamics, Sunnyvale, CA) for fluorescein-labeled probes or exposed to film for DIG-labeled probes. The 3.2 kb pyrG probe contained the entire pyrG coding region, 0.64 kb 5' region, and 1.42 kb 3' untranslated region. A deletant strain was sporulated in RA medium plus 10 mM uridine for approximately 3 days at 28°C, 150 rpm and single spores were isolated using a micromanipulator . Spore isolates were grown on Vogel' s/acetamide agar plus 10 mM uridine. One spore isolate was sporulated in RA medium plus 10 mM uridine. The spore culture was filtered through Miracloth to remove mycelia. Selection for loss of the amdS marker occurred by spreading 9.5 x 10 5 spores of the confirmed pyrG deletant onto each of five 150 mm plates containing fluoroacetamide medium plus 10 mM uridine. These plates were incubated at room temperature for up to 2 weeks. Resulting colonies were subcultured onto COVE plates and fluoroacetamide plates plus lOmM uridine. Colonies that grew well on fluoroacetamide and poorly or not at all on COVE were analyzed further. Spore isolates from three of these strains were subjected to Southern blot analysis. Genomic DNA was cut with FcoRI and probed with the pyrG probe described above. Several of the spore isolates yielded a 1.4 kb hybridizing band indicating an amdS "loop- out" . One spore isolate was chosen and was designated Fusarium venenatum DLM15. Example 6: Expression Levels of Lipase in Independently Grown Fusarium venenatum DLM15 Transformants The pyrG minus Fusarium venenatum strain DLM15 described in Example 5 was transformed with pENI1298 (described in example 2) using standard procedures, as described in Royer et al . 1995, Biotech. 13 1479-1483, replacing acetamide with sodium nitrate (25 mM) . The cells were then cultured on minimal plates at room temp. Transformants appeared after 2 weeks' incubation at a transformation frequency of 50 - 100 /μg DNA, an increase of 10-100 fold over the transformation frequency in the absence of a AMAl sequence. 20 transformants were isolated on minimal media plates and grown for additional 2 weeks at room temp. The transformants were inoculated in a micotiter dish in 200 μl vogel media (as in example 2) either supple- mented with 20 mM uridine or without uridine, and grown at room temp. Transformants supplemented with uridine grew well, but were not able to grow when isolated on minimal media plates without uridine, indicating that the plasmid containing the pyrG gene had been lost, and thus that the plasmid was autonomously replicating. No lipase activity could be detected from transformants grown in uridine supplemented media. Lipase activity could be detected in the media from tranformants growing in minimal media not supplemented with uridine . The average ar- bitrary activity was 34.8 with a relative standard deviation of 15 % when comparing independently grown Fusarium transformants. Thus this plasmid can potentially be used for the creation of libraries in Fusarium species. Example 7: Testing for cotransformation of PCR-fragments The following experiment was carried out, in order to evaluated if multiple PCR-fragments would recombine in vivo with a cut vector (as in example 4) in the same cell, and retain expression of multiple enzymes from the same clone. Two PCR-fragments were made, containing either the lipase gene or the AMG gene (including promoter and terminator) , using standard PCR conditions (94°C, 5 min.; 30 cycles of (94°C, 30 sec; 50°C, 30 sec; 72°C, 1 min.), and 72°C, 5 min.), oligo 115120 (SEQ ID NO:14), oligo 134532 (SEQ ID NO:15) and either a pAHL derivative (lipase variant in pAHL) or a pENI1543 derivate (AMG variant) as template. pENI1543 was constructed by generating a PCR-fragment containing the glucoamylase gene (cDNA) from Aspergillus niger (EMBL: ACX00548 as template) using oligo 139123 (SEQ ID NO:16) and 139124 (SEQ ID N0:17) that was cut with Bglll and Sail and cloned it into pAHL, which had been cut BamHI and Xhol . Approx. 1 μg of each PCR-fragment was transformed into Jal250 (as described in example 4) along with pENI1299 (1 μg) that had been cut Ball and SgrAl and treated with calf intestinal phosphatase . 20 transformants were isolated and inoculated in minimal media (as in example 4) . After 72 hours of incubation the culture media were assayed for the precense of lipase (pnp-butyrate activiy) and glucoamylase (pnp- glucopyranosid activity) . No transformants showed both lipase and glucoamylase activity, thus indicating that cotransformation and expression of multiple variants in the same cell is a very infrequent event, and thus not a problem in the screening of filamentous fungal libraries . Example 8: Construction of pJerS2801 and pHPODl A three piece ligation was performed using a 5.8 kb Bglll/Sphl fragment from pENI1298, a 4.0 kb BssHII/Sphl fragment from pENI1298, and a 0.75 kb BssHI/Bglll fragment from pBANeδ (TAKA/NA2/TPI promoter/AMG terminator; described in WO 98/11203), thus creating pJers2801. A PCR-fragment containing the haloperoxidase from Curvularia verruculosa (patent WO974102-A1) was created using oligo 1029fwp (SEQ ID NO: 18) and 1029rev (SEQ ID NO: 19) . The PCR-fragment was cut Swal and Notl and cloned into pJers2801 cut with the same restriction enzymes, thus creating pHPODl . Example 9 : Expression Levels of haloperoxidase in Independently Grown Transformants pHPODl was transformed into Jal250 as in example 2. Approx. the same transformations frequency was obtained as for pENil298 (example 2) . 10 independent Aspergillus transformants were inoculated into a 96-well microtiter plate containing 200 μl G2-gly-media in each well and grown for 72 hours. Haloper- oxidase activity was determined for each of the 10 transformants essentially as described by De Boer et al [1987; Vanadium containing bromoperoxidase : An example of an oxidoreductase with a high operationel stability in aqueous and organic media, Biotechnol . Bioeng. 30: 607-610]. The average expression level was 82.5 ± 9.1 (arbitrary units). The relative standard deviation in expresion level between independent transformants is approx. 11 %, which is surprisingly low. Thus this invention is not only applicable to lipase, but to other enzymes as well. Example 10: Co-transformation of two plasmids encoding two different enzymes into Jal250 The following experiment was carried out in order to evaluated if two plasmids encoding two different enzymes are transformed and retained in the same cell during growth, and thus can lead to the extracellular co-expression of more than one plasmid encoded enzyme . Jal250 was co-transformed with both 0.1 μg pENI1299 and 0.1 μg pHPODl (as in example 2) . 40 independent transformants were isolated and inoculated in minimal media (as in example 4) . After 72 hours of incubation the culture media was assayed for the precense of lipase (as in example 4) and haloperoxidase activity (as in example 8) . No transformants showed both lipase and haloperoxidase activity indicating that co-transformation and -expression of multiple variants in the same cell is a very infrequent event, and thus not a problem in the screening of filamentous fungal libraries.

Claims: Claims 1. A method of constructing and selecting or screening a library of polynucleotide sequences of interest in filamentous fungal cells wherein the method comprises: (a) transforming the fungal cells with a population of DNA vectors, wherein each vector comprises: (i) a polynucleotide sequence encoding a fungal selection marker and a fungal replication initiating se- quence wherein the marker and the replication initiating sequence do not vary within the population; and (ii) a polynucleotide sequence of interest wherein the population of DNA vectors contains more than one variant of the polynucleotide sequence; (b) cultivating the cells under selection pressure; (c) selecting or screening for one or more transformants expressing a desired characteristic; and (d) isolating the transformant (s) of interest. 2. The method according to claim 1, wherein the library of polynucleotide sequences of interest is prepared by random mutagenesis or naturally occurring allelic variations of at least one parent polynucleotide sequence having or encoding a biological activity or function of interest. 3. The method of claim 1 or 2 , wherein the polynucleotide sequence encodes a polypeptide or is a control sequence; or wherein the polynucleotide sequence encodes a polypeptide or part thereof and further comprises a control sequence in- volved in the expression of the polypeptide or a part of such control sequence . . The method according to claim 3, wherein the polypeptide is a hormone, an enzyme, a receptor or a portion thereof, an antibody or a portion thereof, or a reporter, or a regulatory protein. 5. The method of claim 4, wherein the enzyme is an oxidoreductase, a transferase, a hydrolase, a lyase, an isomerase, or a ligase. 6. The method according to any of claims 4 or 5, wherein the enzyme is an aminopeptidase, amylase, carbohydrase , car- boxypeptidase, catalase, cellulase, chitinase, cutinase, cy- clodextrin glycosyltransferase, deoxyribonuclease, esterase, alpha-galactosidase, beta-galactosidase, glucoamylase, al- pha-glucosidase, beta-glucosidase, invertase, laccase, lipase, mannosidase, mutanase, oxidase, a pectinolytic enzyme, peroxidase, phytase, polyphenoloxidase, a proteolytic enzyme, ribonuclease, transglutaminase, or xylanase . 7. The method according to claim 1 or 2 , wherein the control sequence is an enhancer sequence, a leader sequence, a polyadenylation sequence, a propeptide sequence, a promoter, a replication initiation sequence, a signal sequence, a transcriptional terminator or a translational terminator. 8. The method of claim 7, wherein the promoter is derived from the gene encoding Aspergillus oryzae TAKA amylase, NA2-tpi and Aspergillus niger or Aspergillus awamori glucoamylase. 9. The method according to any of claims 1-8, wherein the selective marker is selected from the group of genes which encode a product which provides for resistance to biocide or viral toxicity, resistance to heavy metal toxicity, or prototrophy to auxotrophs . 10. The method of claim 9, wherein the prototrophy is ob- tained from an enzyme selected from the group of metabolic pathways consisting of nucleotide synthesis, cofactor synthesis, amino acid synthesis, acetamide metabolism, proline metabolism, sulfate metabolism, and nitrate metabolism. 11. The method of claim 9, wherein the selective marker is a gene selected from the group consisting of argB (ornithine carbamoyltransferase) , amdS (acetamidase) , bar (phos- hinothricin acetyltransferase) , hemA (5-aminolevulinate synthase) , hemB (porphobilinogen synthase) , hygB (hygromycin phosphotransferase) , niaD (nitrate reductase) , prn (proline permease) , pyrG (orotidine-5' -phosphate decarboxylase) , py- roA, riboB, sC (sulfate adenyltransferase) , and trpC (anthranilate synthase) . 12. The method of any of claims 1 to 11, wherein the replication initiating sequence is a nucleic acid sequence selected from the group consisting of: (a) a replication initiating sequence having at least 50% identity with the nucleic acid sequence of SEQ ID NO:l or SEQ ID NO : 2 and is capable of initiating replication; (b) a replication initiating sequence which hybridises under low stringency conditions with (i) the nucleic acid sequence of SEQ ID NO : 1 or SEQ ID NO: 2, or (ii) the re- spective complementary strands, wherein the low stringency conditions are defined by prehybridization and hybridization at 42°C in 5x SSPE, 0.3% SDS, 200 mg/ml sheared and denatured salmon sperm DNA, and 25% forma- mide, and wash conditions are defined at 50°C for 30 minutes in 2x SSC, 0.2% SDS; and (c) a subsequence of (a) or (b) , wherein the subse- quence has replication initiating activity. 13. The method of claim 12, wherein the nucleic acid sequence has at least 50% identity, more preferably about 60%, even more preferably about 70%, even more preferably about 80%, even more preferably about 90%, and most preferably about 97% identity with the nucleic acid sequence of SEQ ID NO:l or SEQ ID NO: 2. 14. The method of claim 12, wherein the replication initiat- ing sequence is obtained from a filamentous fungal cell. 15. The method of claim 14, wherein the filamentous fungal cell is a strain of Aspergillus . 16. The method of claim 15, wherein the strain of Aspergillus is obtained from a strain of A . nidulans . 17. The method of any of claims 12 to 16, wherein the replication initiating sequence has the nucleic acid sequence set forth in SEQ ID NO:l or SEQ ID NO:2, or is a respective functional subsequence thereof. 18. The method of claim 2, wherein the modification of the parent polynucleotide sequence is performed by mutagenesis, preferably random mutagenesis, by use of a physical or chemical mutagenizing agent, use of a doped oligonucleotide, DNA shuffling, or by subjecting the nucleic acid sequence to PCR generated mutagenesis, or use of any combination thereof . 19. The method of claim 18, wherein the polynucleotide se- quences of interest are obtained by in vivo recombination between two or more homologous nucleic acid sequences encoding a polypeptide or a regulatory sequence, or any combination of both, comprising: (a) identifying at least one conserved region between the polynucleotide sequences of interest; (b) generating fragments of each of the polynucleotide sequences of interest, wherein said fragments comprise the conserved region (s) of (a); and (c) recombining the fragments of (b) by using the con- served region (s) as (a) homologous linking point (s). 20. The method according to any of claims 1-19, wherein the filamentous fungal cell transformed with the population of DNA vectors is a cell of a strain of Acremonium, Aspergil - lus, Coprinus, Fusarium, Humicola , Mucor, Myceliopthora, Neurospora, Penicillium, Thielavia , Tolypocladium or Trichoderma . 21. The method according to claim 20, wherein the cell is an Aspergillus oryzae, Aspergillus niger, Aspergillus nidulans, Coprinus cinereus, Fusarium oxysporum, or Trichoderma reesei cell. 22. A method of constructing and screening or selecting a li- brary of polynucleotide sequences of interest in a filamentous fungal cell, wherein the method comprises: (a) transforming a culture of bacterial or yeast cells with a population of the DNA vectors as described in any of claims 1-21, wherein the vector further comprises a nucleic acid sequence encoding a bacterial or yeast se- lective marker and a bacterial or yeast replication initiating sequence; (b) cultivating the bacterial or yeast cells under selection pressure; (c) isolating the DNA constructs from the transformants of (b) ; (d) transforming filamentous fungal cells with the DNA constructs of (c) ; (e) cultivating the filamentous fungal cells of (d) ; (f) selecting or screening for one or more filamentous fungal transformants expressing a desired characteristic; and (g) isolating the filamentous fungal transformant (s) of interest . 23. The method of claim 22, wherein the bacterial or yeast selective marker is selected from the group of genes which encode a product which provides for resistance to biocide or viral toxicity, resistance to heavy metal toxicity, or prototrophy to auxotrophs . 24. Use of a fungal replication initiating sequence in the construction of a library of polynucleotide sequences of interest . 25. The use according to claim 24, wherein the fungal replication initiating sequence is a nucleic acid sequence selected from the group consisting of: (a) a replication initiating sequence having at least 50% identity with the nucleic acid sequence of SEQ ID NO:l or SEQ ID NO: 2 and is capable of initiating replication; (b) a replication initiating sequence which hybridises under low stringency conditions with (i) the nucleic acid sequence of SEQ ID NO:l or SEQ ID NO: 2, or (ii) the respective complementary strands, wherein the low stringency conditions are defined by prehybridization and hy- bridization at 42°C in 5x SSPE, 0.3% SDS, 200 mg/ml sheared and denatured salmon sperm DNA, and 25% forma- mide, and wash conditions are defined at 50°C for 30 minutes in 2x SSC, 0.2% SDS; and (c) a subsequence of (a) or (b) , wherein the subse- quence has replication initiating activity. 26. The use according to claim 25, wherein the replication initiating sequence is obtained from a filamentous fungal cell, in particular from a strain of Aspergillus, such as A. nidulans . 27. The use according to claim 25 or 26, wherein the replication initiating sequence has the nucleic acid sequence set forth in SEQ ID NO:l or SEQ ID NO:2, or is a respective functional subsequence thereof. 28. A library of polynucleotide sequences of interest which library comprises filamentous fungal cells tranformed with a population of DNA vectors, wherein each vector comprises: (i) a gene encoding a fungal selection marker and a fungal replication initiating sequence wherein the marker and the replication initiating sequence do not vary within the population; and (ii) a polynucleotide sequence of interest wherein the population of DNA vectors contains more than one vari- ant of the polynucleotide sequence. 29. The library according to claim 28, wherein the vector further comprises a nucleic acid sequence encoding a bacterial or yeast selective marker and a bacterial or yeast rep- lication initiating sequence.

Inventors: VIND JESPER, VIND, JESPER
Applicants: NOVONORDISK AS, NOVO NORDISK A/S
Application year: 1999
Main classification: C12N15
IPC codes: C12N1, C12N9, C12N15, C12Q1