Mapping the heterogeneous DNA region that determines the nine A alpha mating-type specificities of Schizophyllum...

Classical genetic studies identified nine mating-type specificities at the A alpha locus of the Basidiomycete fungus Schizophyllum commune. We have used Southern blot hybridizations to generate EcoRI restriction maps of the A alpha locus for 18 strains, including all nine specificities. A alpha 1, A alpha 3 and A alpha 4 DNA was subcloned from three cosmids and used as probes. A unique region of DNA was found for each of the three cloned specificities. Hybridization was detected in this region only if the probe(s) and the blotted genomic DNAs were from strains with the same A alpha specificity. DNAs from strains with the same A alpha specificity hybridize regardless of geographic origin, but DNAs from strains with different A alpha specificities do not cross-hybridize. The results demonstrate two size classes of unique A alpha DNA. This unique DNA is about 4.5 kb in A alpha 1 strains and about 7.0-8.5 kb in other strains. Transcription regulators Z and Y, which were deduced previously from the DNA sequence of the A alpha 1, A alpha 3 and A alpha 4 loci, are probably encoded by all non-A alpha 1 loci. The smaller A alpha 1 loci appear to encode only Y and lack sequence for Z. No evidence was found for a locus that encodes only Z. The lack of hybridization detected between A alpha loci with different specificities suggests that the evolution of A alpha has resulted from extensive sequence divergence.