Isolation and characterisation of the B42 mating type locus of Coprinus cinereus
C. cinereus, any two of which are sufficient to promote B-regulated development following cell fusion. The isolation of the B42 locus is described along with the DNA sequence analysis that identified nine B mating type genes within a 27kb B42 -specific DNA sequence. Six of the genes, with small transcripts of 800-900nt, encode the mating pheromone precursors and the other three, with 1.9 to 2-5kb transcripts, encode the transmembrane pheromone receptors. The genes are arranged in three groups, designated group 1, 2 and 3, each consisting of one receptor gene and two pheromone genes. B42 and B6 share the same alleles of the group 1 genes, but not those of groups 2 and 3. This was demonstrated by DNAsequence analysis and Southern blot analysis. None of the group 1 genes from B42 were able to activate B -regulated development in a B6 host when introduced by transformation but with one exception, all genes from group 2 and group 3 were able to do so. This analysis led to the recognition that the three genes in any one group are held together in an allele-specific DNA sequence and that Southern blot analysis and transformation can be used to identify shared alleles in uncloned loci. Extensive Southern analyses using cloned genes to probe genomic DNAs from strains having other B mating specificites showed that different B loci may share identical alleles of two groups of genes. Mating partners thus require different alleles of only one group of genes to generate a compatible B mating interaction. Transformation analyses with the same cloned genes confirmed the conclusions derived from the hybridisation data. Multiple B mating specificities thus appear to be derived from three groups of multiallelic and functionally redundant genes. A tenth gene located within the B42- specific DNA sequence encodes a putative transporter protein belonging to the major facilitator superfamily (MFS). In other genomic backgrounds this gene lies in homologous flanking sequences and its presence within the B42 locus is unlikely to be related to mating type function.