Molecular cloning and characterization of the met2 gene from Ascobolus immersus

Novel relationships among DNA methylation, histone modifications and gene expression in Ascobolus

By studying Ascobolus strains methylated in various portions of the native met2 gene or of the hph transgene, we generalized our previous observation that methylation of the downstream portion of a gene promotes its stable silencing and triggers the production of truncated transcripts which rarely extend through the methylated region. In contrast, methylation of the promoter region does not promote efficient gene silencing. The chromatin state of met2 methylated strains was investigated after partial micrococcal nuclease (MNase) digestion. We show that MNase sensitive sites present along the unmethylated regions are no longer observed along the methylated ones. These chromatin changes are not resulting from the absence of transcription. They are associated, in both met2 and hph, with modifications of core histones corresponding, on the N terminus of histone H3, to an increase of dimethylation of lysine 9 and a decrease of dimethylation of lysine 4. Contrary to other organisms, these changes are independent of the transcriptional state of the genes, and furthermore, no decrease in acetylation of histone H4 is observed in silenced genes.

Functional analysis of the homoserine O-acetyltransferase gene and its identification as a selectable marker in Gibberella zeae

We used restriction enzyme-mediated integration (REMI) to identify a methionine auxotrophic mutant of Gibberella zeae, an important cereal pathogen. In addition to its methionine requirement, the G. zeae REMI mutant designated Z43R3912 showed pleiotropic phenotypes, including reduced virulence on host plants and lack of sexual development. Outcrossing of Z43R3912 with a mat1-1 deletion strain confirmed that the mutation of Z43R3912 was tagged with the hygromycin B resistance marker. The vector insertion site in Z43R3912 was identified within the ORF designated GzmetE, encoding a putative homoserine O-acetyltrasferase (HOA). Gene disruption analyses confirmed that GzmetE was responsible for the pleiotropic phenotypes of Z43R3912. Genetic complementation of the G. zeae methionine auxotroph with an intact copy of the Aspergillus nidulans metE and GzmetE genes suggests that the HOA gene can be used as a selectable marker for transformation of G. zeae.

The bacteriophage T4 anti-sigma factor AsiA is not necessary for the inhibition of early promoters in vivo

Bacteriophage T4 early promoters are utilized immediately after infection and are abruptly turned off 2-3 min later (at 30 degrees C) when the middle promoters are activated. The viral early protein AsiA has been suspected to bring about this transcriptional switch: not only does it activate transcription at middle promoters in vivo and in vitro but it also shows potent anti-sigma70 activity in vitro, suggesting that it is responsible for the shut-off of early transcription. We show here that after infection with a phage deleted for the asiA gene the inhibition of early transcription occurs to the same extent and with the same kinetics as in a wild-type infection. Thus, another AsiA-independent circuit efficiently turns off early transcription. The association of a mutation in asiA with a mutation in mod, rpbA, motA or motB has no effect on the inhibition of early promoters, showing that none of these phage-encoded transcriptional regulators is necessary for AsiA-independent shut-off. It is not known whether AsiA is able to inhibit early promoters in vivo, but host transcription is strongly inhibited in vivo upon induction of AsiA from a multicopy plasmid.

Histone H1 is dispensable for methylation-associated gene silencing in Ascobolus immersus and essential for long life span

A gene encoding a protein that shows sequence similarity with the histone H1 family only was cloned in Ascobolus immersus. The deduced peptide sequence presents the characteristic three-domain structure of metazoan linker histones, with a central globular region, an N-terminal tail, and a long positively charged C-terminal tail. By constructing an artificial duplication of this gene, named H1, it was possible to methylate and silence it by the MIP (methylation induced premeiotically) process. This resulted in the complete loss of the Ascobolus H1 histone. Mutant strains lacking H1 displayed normal methylation-associated gene silencing, underwent MIP, and showed the same methylation-associated chromatin modifications as did wild-type strains. However, they displayed an increased accessibility of micrococcal nuclease to chromatin, whether DNA was methylated or not, and exhibited a hypermethylation of the methylated genome compartment. These features are taken to imply that Ascobolus H1 histone is a ubiquitous component of chromatin which plays no role in methylation-associated gene silencing. Mutant strains lacking histone H1 reproduced normally through sexual crosses and displayed normal early vegetative growth. However, between 6 and 13 days after germination, they abruptly and consistently stopped growing, indicating that Ascobolus H1 histone is necessary for long life span. This constitutes the first observation of a physiologically important phenotype associated with the loss of H1.

Analysis of the methionine biosynthetic pathway in the extremely thermophilic eubacterium Thermus thermophilus

Four DNA fragments that could rescue the mutations of four Met- mutants were cloned from Thermus thermophilus HB27 and their complete nucleotide sequences were determined. Two of the four fragments respectively contained the greater parts of the metF and metH genes, the predicted amino acid sequences of which showed identities of 30.8% and 32.7% with 5,10-methylenetetrahydrofolate reductase (EC 1.7.99.5) and vitamin B12-dependent homocysteine transmethylase (EC 2.1.1.13) of Escherichia coli. The other two DNA fragments, which overlapped one another, contained two open reading frames whose predicted amino acid sequences were respectively similar to those of O-acetylhomoserine sulfhydrylase (EC 4.2.99.10, the product of the MET17 gene) and homoserine O-acetyltransferase (EC 2.3.1.31, the product of the MET2 gene) of Saccharomyces cerevisiae. The metF, metH, MET2, and MET17 genes of T. thermophilus were disrupted by introducing the heat-stable kanamycin nucleotidyltransferase gene into the genome. Each transformant showed methionine auxotrophy. Both the MET2- and MET17-disrupted mutants could grow in a minimal medium containing homocysteine but not in the same medium containing succinylhomoserine or cystathionine. In contrast, the metF- and metH-disrupted mutants could not grow in the minimal medium containing homocysteine. These results suggest that in T. thermophilus, homoserine is directly converted to homocysteine via O-acetylhomoserine and that homocysteine is methylated to synthesize methionine.

Disruption of TRI101, the gene encoding trichothecene 3-O-acetyltransferase, from Fusarium sporotrichioides

We screened a Fusarium sporotrichioides NRRL 3299 cDNA expression library in a toxin-sensitive Saccharomyces cerevisiae strain lacking a functional PDR5 gene. Fourteen yeast transformants were identified as resistant to the trichothecene 4,15-diacetoxyscirpenol, and each carried a cDNA encoding the trichothecene 3-O-acetyltransferase that is the F. sporotrichioides homolog of the Fusarium graminearum TRI101 gene. Mutants of F. sporotrichioides NRRL 3299 produced by disruption of TRI101 were altered in their abilities to synthesize T-2 toxin and accumulated isotrichodermol and small amounts of 3, 15-didecalonectrin and 3-decalonectrin, trichothecenes that are not observed in cultures of the parent strain. Our results indicate that TRI101 converts isotrichodermol to isotrichodermin and is required for the biosynthesis of T-2 toxin.

Masc2, a gene from Ascobolus encoding a protein with a DNA-methyltransferase activity in vitro, is dispensable for in vivo methylation

We have shown previously that masc1, a gene encoding a putative C5-DNA-methyltransferase (MTase), was necessary for the de novo 'Methylation Induced Premeiotically' (MIP) process and sexual reproduction in Ascobolus, whereas it was dispensable for maintenance methylation. A second MTase gene from Ascobolus, masc2, encodes a protein, Masc2, which possesses the large amino-terminal part characteristic of eukaryotic maintenance MTases. In vitro assays have shown that Masc2 displays a methylation activity, suggesting that it might be the MTase responsible for maintenance methylation. To check its function in vivo, we engineered a disruption of the masc2 gene. The resulting mutant strains did not exhibit any particular phenotype during either vegetative growth or sexual reproduction. Neither the masc2 mutation nor the double masc1 masc2 mutation had any detectable effect upon the maintenance of the pre-existing methylation of single gene copies previously subjected to MIP, natural retroelement-like repeats and tandemly repeated rDNA. The masc2 mutation did not alter either MIP or the other de novo methylation process that operates in vegetatives cells. Nor did it impair the meiotic process of methylation transfer. These results suggest that at least a third MTase gene responsible for maintenance and vegetative de novo methylation is present in Ascobolus.

Molecular characterization and sequence of a methionine biosynthetic locus from Pseudomonas syringae

Two methionine biosynthetic genes in Pseudomonas syringae pv. syringae, metX and metW, were isolated, sequenced, and evaluated for their roles in methionine biosynthesis and bacterial fitness on leaf surfaces. The metXW locus was isolated on a 1.8-kb DNA fragment that was required for both methionine prototrophy and wild-type epiphytic fitness. Sequence analysis identified two consecutive open reading frames (ORFs), and in vitro transcription-translation experiments provided strong evidence that the ORFs encode proteins with the predicted molecular masses of 39 and 22.5 kDa. The predicted amino acid sequence of MetX (39 kDa) showed homology to several known and putative homoserine O-acetyltransferases. This enzyme is the first enzyme in the methionine biosynthetic pathway of fungi, gram-negative bacteria of the genus Leptospira, and several gram-positive bacterial genera. Both metX and metW were required for methionine biosynthesis, and transcription from both genes was not repressed by methionine. MetW (22.5 kDa) did not show significant homology to any known protein, including prokaryotic and eukaryotic methionine biosynthetic enzymes. Several classes of methionine auxotrophs, including metX and metW mutants, exhibit reduced fitness on leaf surfaces, indicating a requirement for methionine prototrophy in wild-type epiphytic fitness. This requirement is enhanced under environmentally stressful conditions, suggesting a role for methionine prototrophy in bacterial stress tolerance.

Metabolism of sulfur amino acids in Saccharomyces cerevisiae

Sulfur amino acid biosynthesis in Saccharomyces cerevisiae involves a large number of enzymes required for the de novo biosynthesis of methionine and cysteine and the recycling of organic sulfur metabolites. This review summarizes the details of these processes and analyzes the molecular data which have been acquired in this metabolic area. Sulfur biochemistry appears not to be unique through terrestrial life, and S. cerevisiae is one of the species of sulfate-assimilatory organisms possessing a larger set of enzymes for sulfur metabolism. The review also deals with several enzyme deficiencies that lead to a nutritional requirement for organic sulfur, although they do not correspond to defects within the biosynthetic pathway. In S. cerevisiae, the sulfur amino acid biosynthetic pathway is tightly controlled: in response to an increase in the amount of intracellular S-adenosylmethionine (AdoMet), transcription of the coregulated genes is turned off. The second part of the review is devoted to the molecular mechanisms underlying this regulation. The coordinated response to AdoMet requires two cis-acting promoter elements. One centers on the sequence TCACGTG, which also constitutes a component of all S. cerevisiae centromeres. Situated upstream of the sulfur genes, this element is the binding site of a transcription activation complex consisting of a basic helix-loop-helix factor, Cbf1p, and two basic leucine zipper factors, Met4p and Met28p. Molecular studies have unraveled the specific functions for each subunit of the Cbf1p-Met4p-Met28p complex as well as the modalities of its assembly on the DNA. The Cbf1p-Met4p-Met28p complex contains only one transcription activation module, the Met4p subunit. Detailed mutational analysis of Met4p has elucidated its functional organization. In addition to its activation and bZIP domains, Met4p contains two regulatory domains, called the inhibitory region and the auxiliary domain. When the level of intracellular AdoMet increases, the transcription activation function of Met4 is prevented by Met30p, which binds to the Met4 inhibitory region. In addition to the Cbf1p-Met4p-Met28p complex, transcriptional regulation involves two zinc finger-containing proteins, Met31p and Met32p. The AdoMet-mediated control of the sulfur amino acid pathway illustrates the molecular strategies used by eucaryotic cells to couple gene expression to metabolic changes.

Homoserine O-acetyltransferase, involved in the Leptospira meyeri methionine biosynthetic pathway, is not feedback inhibited

The Leptospira meyeri serovar semaranga metX gene was identified by complementation of an Escherichia coli metA mutant, i.e., devoid of homoserine O-succinyltransferase. However, the MetX protein exhibited a homoserine O-acetyltransferase activity in agreement with its similarity to homoserine O-acetyltransferases. Reverse transcription-PCR analysis demonstrated that metX is the second gene of an operon.

Native DNA repeats and methylation in Ascobolus

We identified two classes of native dispersed DNA repeats in the Ascobolus genome. The first class consisted of several kilobase long, methylated repeats. These repeats, named Mars (methylated Ascobolus repeated sequences), fell in one family of LINE-like elements and in three families of LTR-containing retrotransposable elements. The methylation features of Mars elements were those expected if they were natural targets for the MIP (methylation induced premeiotically) previously discovered in Ascobolus. The second class consisted of short repeats, approximately 100 bp long, corresponding to 5S rRNA and tRNA genes. As expected from their size, which was too small to allow MIP to occur, they were unmethylated, as were 26 kb of unique sequences tested. These observations are consistent with the hypothesis that MIP is targeted at natural DNA repeats and constitutes a defensive process against the detrimental consequences of the spreading of mobile elements throughout the genome. The 9 kb tandem repeats harbouring the 28S, 18S and 5.8S rRNA genes displayed methylation features suggesting that rDNA methylation proceeds through a process other than MIP.

Methylation of DNA repeats of decreasing sizes in Ascobolus immersus

In Ascobolus immersus, DNA duplications are subject to the process of methylation induced premeiotically (MIP), which methylates the cytosine residues within the repeats and results in reversible gene silencing. The triggering of MIP requires pairing of the repeats, and its detection requires maintenance of the resulting methylation. MIP of kilobase-size duplications occurs frequently and leads to the methylation of all C residues in the repeats, including those belonging to non-CpG sequences. Using duplications of decreasing sizes, we observed that tandem repeats never escaped MIP when larger than 630 bp and showed a sudden and drastic drop in MIP frequencies when their sizes decreased from 630 to 317 bp. This contrasted with the progressive decrease of MIP frequencies observed with ectopic repeats, in which apparently the search for homology influences the MIP triggering efficiency. The minimal size actually required for a repeat to undergo detectable MIP was found to be close to 300 bp. Genomic sequencing and Southern hybridization analyses using restriction enzymes sensitive to C methylation showed a loss of methylation at non-CpG sites in short DNA segments, methylation being restricted to a limited number of CpG dinucleotides. Our data suggest the existence of two distinct mechanisms underlying methylation maintenance, one responsible for methylation at CpG sites and the other responsible for methylation at non-CpG sites.

Cloning and sequence of the Ascobolus immersus S-adenosyl-L-methionine synthetase-encoding gene

The structural gene encoding S-adenosyl-L-methionine synthetase (SAM-S) in the fungus Ascobolus immersus has been cloned and sequenced. It contains a 1179-bp ORF, interrupted by three introns, encoding a 393-amino-acid protein (42 978 Da) that is 90% homologous to the SAM-S of the filamentous fungus Neurospora crassa, indicating that these fungi are closely related species.

Isolation and characterization of Tri3, a gene encoding 15-O-acetyltransferase from Fusarium sporotrichioides

An acetyltransferase gene (Tri3) was isolated from Fusarium sporotrichioides by complementation of a previously identified Tri3- mutant and shown to be closely linked to three other trichothecene biosynthetic pathway genes. Comparison of the Tri3 sequence with its cDNA revealed the presence of four introns. The Tri3 cDNA contains a 1,539-bp open reading frame that encodes a protein with a molecular mass of 57,418 Da. Regulation of Tri3 transcription in liquid cultures appeared identical to that of other trichothecene pathway genes. Disruption of the Tri3 gene resulted in the accumulation of deacetylated calonectrins rather than T-2 toxin. The results of whole-cell feeding experiments with Tri3- strains suggested that 15-O-acetylation is blocked. Cell-free feeding experiments confirmed that Tri3- strains are able to acetylate a trichothecene C-3 hydroxyl group but are unable to acetylate a trichothecene C-15 hydroxyl group. Our results show that Tri3 encodes an acetyltransferase that converts 15-decalonectrin to calonectrin.

Stereochemical analysis of the antigenic tip of the V3 loop peptide of HIV-1 gp120

A novel multiple turn conformation has been observed for a segment GPGRAFY in the crystal structure of a complex of HIV-1 gp120 V3 loop peptide with the Fab fragment of a neutralizing antibody [Ghiara et al. (1994) Science 264, 82-85]. A structural motif has been defined for the peptide segment, employing idealized backbone conformations characterized by ranges of virtual C alpha torsion angles and bond angles. A search of 122 high-resolution protein crystal structures has permitted identification of 24 examples of similar structural motifs. Two major conformational families have been identified, which differ primarily in the conformation at residue 3. The observed conformation at residue 3 in family 1 is left-handed helical (alpha L) and that in family 2 is right-handed helical (alpha R). Of the 10 examples in family 1, 9 examples have Gly residues at position 3. Of the 12 examples in family 2, 7 examples have Asn/Asp at position 3. Computer modeling of the V3 loop tip sequence using the two backbone conformational families as starting points leads to minimum-energy conformations in which antigenically important side-chains occupy similar spatial arrangements. This stereochemical analysis of the V3 loop tip sequence suggests a rational basis for the design of synthetic analog peptides for use as viral antagonists or synthetic antigens.

Clustering of multiple transgene integrations in highly-unstable Ascobolus immersus transformants

A large proportion of Ascobolus immersus transformants are highly unstable in crosses: the phenotype conferred by the transgene is not transmitted to the progeny, irrespective of the endogenous or foreign origin of the transgene. They all have integrated multiple transgene copies, clustered at a single chromosomal site or at tightly-linked sites. Clustered non-homologous integrations are always rearranged. Yet they never escape the "methylation induced premeiotically" (MIP) process. This always results in gene silencing, even when the transgene is partially repeated, accounting for the high instability of these transformants.

Methylation induced premeiotically in Ascobolus: coextension with DNA repeat lengths and effect on transcript elongation

The effect of duplications of gene fragments on the triggering of DNA methylation induced premeiotically (MIP) was studied in the sexual progeny of strains harboring, in addition to the resident met2 gene, a fragment of this gene inserted at an ectopic position. Cytosine methylation of the resident gene was checked for each of the eight duplications tested. Methylation was always found and it was coextensive with the length of the duplications. Silencing of the resident gene was triggered by duplications of segments corresponding to the region 5' to the open reading frame, to only the open reading frame, or to segments beginning 0.87-1.2 kb downstream from the transcription start. Silencing was accompanied by either the absence of transcripts or the presence of truncated transcripts, which suggests that methylation acts on transcript elongation.

The cefG gene of Cephalosporium acremonium is linked to the cefEF gene and encodes a deacetylcephalosporin C acetyltransferase closely related to homoserine O-acetyltransferase

The gene (cefG) encoding the acetyl coenzyme A:deacetylcephalosporin C acetyltransferase of Cephalosporium acremonium (synonym Acremonium chrysogenum) C10 has been cloned. It contains two introns and encodes a protein of 444 amino acids with an M(r) of 49,269 that correlates well with the M(r) deduced by gel filtration. The cefG gene is linked to the cefEF gene (encoding the bifunctional deacetoxycephalosporin C synthase/hydroxylase), but it is expressed in an orientation opposite that of the cefEF gene. Two transcripts of 1.2 and 1.4 kb were found in C. acremonium that correspond to the cefEF and cefG genes, respectively; the degree of expression of the cefG gene was clearly lower than that of the cefEF gene in 48-h cultures. The cloned cefG complemented the deficiency of deacetylcephalosporin acetyltransferase in the nonproducer mutant C. acremonium ATCC 20371 and restored cephalosporin biosynthesis in this strain. Heterologous expression of the cefG genes took place in Penicillium chrysogenum. The deacetylcephalosporin acetyltransferase showed a much higher degree of homology with the O-acetylhomoserine acetyltransferases of Saccharomyces cerevisiae and Ascobolus immersus than with other O-acetyltransferases. The cefEF-cefG cluster of genes encodes the enzymes that carry out the three late steps of the cephalosporin biosynthetic pathway and is not linked to the pcbAB-pcbC gene cluster that encodes the first two steps of the pathway.

Molecular cloning of acetyl coenzyme A: deacetylcephalosporin C o-acetyltransferase cDNA from Acremonium chrysogenum: sequence and expression of catalytic activity in yeast

Acetyl CoA: deacetylcephalosporin C o-acetyltransferase(DCPC-ATF) catalyses the final step in the biosynthesis of cephalosporin C, the conversion of deacetylcephalosporin C to cephalosporin C. A cDNA encoding DCPC-ATF has been isolated from a cDNA library of a cephalosporin C producing fungus Acremonium chrysogenum using oligonucleotide probes based on N-terminal amino acid sequences of the enzyme. The cDNA contains a single large open reading frame for a putative precursor consisting of 12 amino acid(AA) leader peptide of unknown function, 274 AA large subunit and 126 AA small subunit at the carboxyl end. The cDNA was expressed in yeast exhibiting a functional DCPC-ATF activity. It was also indicated that the leader peptide was not essential for expression of the enzyme activity. The primary structure of DCPC-ATF shows significant homology with those of acetyl CoA: homoserine o-acetyltransferase in Saccharomyces cerevisiae and Ascobolas immersus.

Targeted transformation of Ascobolus immersus and de novo methylation of the resulting duplicated DNA sequences

To develop a method to modify genomic sequences in Ascobolus immersus by precisely reintroducing defined DNA segments previously manipulated in vitro, we investigated the effect of transforming DNA conformation on recombination with chromosomal sequences. Circular single-stranded DNA carrying the met2 gene and double-stranded DNA linearized by cutting within the met2 gene both transformed protoplasts of a met2 mutant strain of A. immersus to prototrophy. In contrast to the equivalent circular double-stranded DNA, which chiefly integrated at nonhomologous chromosomal sites, single-stranded and double-stranded cut DNAs recombined primarily with the homologous chromosomal met2 sequence. Of the single-stranded DNA transformants, 65% resulted from replacement of the resident met2 mutation by the exogenous wild-type allele. In 70% of the double-stranded-cut DNA transformants, one or more copies of the transforming DNA had integrated at the met2 locus, leading to tandem duplications of the met2 target region separated by plasmid DNA. These duplicated sequences could recombine, leading to progeny containing only one copy of the met2 region. This resulted in a precise gene replacement if the wild-type allele had been retained. In addition, we show that newly duplicated sequences were most often de novo methylated at the cytosine residues during the sexual phase. Cytosine methylation was associated with inactivation of the integrated met2 gene(s) in segregants of crosses. However, methylation was not accurately maintained at each DNA replication cycle, so that Met- segregants recovered a wild-type phenotype through successive mitotic divisions. This finding indicated that met2 genes were silenced by methylation alone.

In organello replication and viral affinity of linear, extrachromosomal DNA of the ascomycete Ascobolus immersus

Linear, extrachromosomal DNA's of the filamentous fungus Ascobolus immersus are localized within the mitochondria. These linear plasmids have no homology to the high molecular weight mtDNA (hmw mtDNA). For analysis of plasmid replication an in organello DNA synthesis system was developed, in which radionucleotides were incorporated into intact mitochondria. Plasmid DNA is labelled preferentially in this system. From replication analysis of a specific plasmid there is evidence of a virus-like protein-primed replication. Sequence analysis of this plasmid reveals that a viral DNA polymerase is encoded. Thus, these genetic elements presumably are viral remnants rather than true plasmids.

Targeted transformation of Ascobolus immersus and de novo methylation of the resulting duplicated DNA sequences

To develop a method to modify genomic sequences in Ascobolus immersus by precisely reintroducing defined DNA segments previously manipulated in vitro, we investigated the effect of transforming DNA conformation on recombination with chromosomal sequences. Circular single-stranded DNA carrying the met2 gene and double-stranded DNA linearized by cutting within the met2 gene both transformed protoplasts of a met2 mutant strain of A. immersus to prototrophy. In contrast to the equivalent circular double-stranded DNA, which chiefly integrated at nonhomologous chromosomal sites, single-stranded and double-stranded cut DNAs recombined primarily with the homologous chromosomal met2 sequence. Of the single-stranded DNA transformants, 65% resulted from replacement of the resident met2 mutation by the exogenous wild-type allele. In 70% of the double-stranded-cut DNA transformants, one or more copies of the transforming DNA had integrated at the met2 locus, leading to tandem duplications of the met2 target region separated by plasmid DNA. These duplicated sequences could recombine, leading to progeny containing only one copy of the met2 region. This resulted in a precise gene replacement if the wild-type allele had been retained. In addition, we show that newly duplicated sequences were most often de novo methylated at the cytosine residues during the sexual phase. Cytosine methylation was associated with inactivation of the integrated met2 gene(s) in segregants of crosses. However, methylation was not accurately maintained at each DNA replication cycle, so that Met- segregants recovered a wild-type phenotype through successive mitotic divisions. This finding indicated that met2 genes were silenced by methylation alone.

The ura5 gene of the ascomycete Sordaria macrospora: molecular cloning, characterization and expression in Escherichia coli

We cloned the ura5 gene coding for the orotate phosphoribosyl transferase from the ascomycete Sordaria macrospora by heterologous probing of a Sordaria genomic DNA library with the corresponding Podospora anserina sequence. The Sordaria gene was expressed in an Escherichia coli pyrE mutant strain defective for the same enzyme, and expression was shown to be promoted by plasmid sequences. The nucleotide sequence of the 1246-bp DNA fragment encompassing the region of homology with the Podospora gene has been determined. This sequence contains an open reading frame of 699 nucleotides. The deduced amino acid sequence shows 72% similarity with the corresponding Podospora protein.

Stable allele replacement and unstable non-homologous integration events during transformation of Ascobolus immersus

An homologous transformation system for the filamentous fungus Ascobolus immersus has been developed, based on the complementation of a met2 mutation by the wild-type (wt) allele gene encoding homoserine O-transacetylase. Transformation of A. immersus met2 mutants occurs with moderate frequencies (about 50 transformants per microgram input DNA). Analysis of the DNA of the met2+ transformants showed that transformation resulted either in a single integration of the donor DNA into the genome by many different nonhomologous recombination events or in the substitution of the endogenous met2 mutation by the wt transforming allele. The relative frequencies of both events depended on the vector sequences carrying the cloned met2 gene. Whereas the substitution event led, as expected, to genetically stable transformants, the non-homologous integration was always associated with a strong instability when transformants were crossed and underwent meiosis.