Three subfamilies of pheromone and receptor genes generate multiple B mating specificities in the mushroom Coprinus cinereus

The B mating type locus of the basidiomycete Coprinus cinereus encodes a la rge family of lipopeptide pheromones and their seven transmembrane domain r eceptors. Here we show that the B42 locus, like the previously described BG locus, derives its unique specificity from nine multiallelic genes that ar e organized into three subgroups each comprising a receptor and two pheromo ne genes. We show that the three genes within each group are kept together as a functional unit by being embedded in an allele specific DNA sequence. Using a combination of sequence analysis, Southern blotting, and DNA-mediat ed transformation with cloned genes, we demonstrate that different B loci m ay share alleles of one or two groups of genes. This is consistent with the prediction that the three subgroups of genes are functionally redundant an d that it is the different combinations of their alleles that generate the multiple B mating specificities found in nature. The B42 locus was found to contain an additional gene, mfs1, that encodes a putative multidrug transp orter belonging to the major facilitator family. In strains with other B ma ting specificities, this gene, whose functional significance was nor establ ished, lies in a region of shared homology flanking the B locus.