MAPPING THE HETEROGENEOUS DNA REGION THAT DETERMINES THE 9-A-ALPHA-MATING-TYPE SPECIFICITIES OF SCHIZOPHYLLUM-COMMUNE

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 restrictio n maps of the Act locus for 18 strains, including all nine specificiti es. A alpha 1, A alpha 3 and A alpha 4 DNA was subcloned from three co smids and used as probes. A unique region of DNA was found for each of the three cloned specificities. Hybridization was detected in this re gion only if the probe(s) and the blotted genomic DNAs were from strai ns with the same A alpha specificity. DNAs from strains with the same A alpha specificity hybridize regardless of geographic origin, but DNA s from strains with different A alpha specificities do not cross-hybri dize. 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 d educed previously from the DNA sequence of the A alpha 1, A alpha 3 an d 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 tvas found for a locus that encodes only Z. The lack of hybridization detected between Act loci with different specificities s uggests that the evolution of Act has resulted from extensive sequence divergence.