Molecular analysis of the Coprinus cinereus mating type A factor demonstrates an unexpectedly complex structure

We report here the molecular cloning of the A43 mating type factor from Coprinus cinereus, a basidiomycetous fungus. Our molecular analyses revealed an unexpected source of variation in the A factor. Though genetic studies have demonstrated that A has two subunits, alpha and beta, we located three nonoverlapping fragments in the A43 region that have A factor function following DNA-mediated transformation. The three fragments demonstrate no similarity to one another as judged by restriction enzyme maps and by hybridization on Southern blots. We conclude that the A43 factor is composed of at least three subunits. When strains carrying different A factors are examined by hybridization to the cloned subunits, extensive polymorphism is seen. Both intensity of hybridization and restriction fragment lengths vary between strains. Some strains fail to show any hybridization to a probe. In contrast, other strains from widely separated geographic locations apparently share very similar subunits. From comparative restriction enzyme mapping of A43 and a mutated A43 factor, we inferred that a 12-kb deletion in the A factor was responsible for the constitutive, dominant phenotype of the mutated A factor. The results of transformation experiments support an activator model for the activity of the A factor in regulating the A pathway.

Targeted transformation in Coprinus cinereus

We examined the influence of DNA form and size on the arrangement and genomic location of transforming DNA sequences in the basidiomycete Coprinus cinereus. Protoplasts with either single or double mutations in the tryptophan synthetase (TRP1) gene were transformed with cloned copies of this gene which contained only a single DNA strand, contained a specific single nick within the C. cinereus sequences (4.8 kb), contained a specific double-strand break, or contained an additional 35 kb of flanking genomic sequences. Gene replacement events were recovered when each DNA type was used. However, none of these substrates offers a substantial improvement in transformation or targeting frequency when compared to supercoiled circular DNA, which has allowed recovery of both gene replacements as well as homologous insertions in 5% of the transformants analyzed. The frequency of transformants carrying tandem insertions with multiple copies of the transforming DNA was reduced when single-stranded DNA was used, and increased when DNA containing double-strand breaks was used. These results have important implications for the efficient design of targeted transformation and co-transformation experiments.

Duplications created by transformation in Sordaria macrospora are not inactivated during meiosis

We present here the first report of a transformation system developed for the filamentous fungus Sordaria macrospora. Protoplasts from a ura-5 strain were transformed using the cloned Sordaria gene at a frequency of 2 x 10(-5) transformants per viable protoplast (10 per microgram of DNA). Transformation occurred by integration of the donor sequences in the chromosomes of the recipient strain. In 71 cases out of 74, integration occurred outside the ura5 locus; frequently several (two to four) copies were found at a unique integration site. Using the advantage of the spore colour phenotype of the ura5-1 marker, we have shown that the transformed phenotype is stable through mitosis and meiosis in all transformants analysed. No methylation of the duplicated sequences could be observed during meiotic divisions in the transformants.

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.