Only one of the paired Schizophyllum commune A alpha mating-type, putative homeobox genes encodes a homeodomain essential for A alpha-regulated development.

The A alpha mating-type locus is one of four master regulatory loci controlling sexual development in Schizophyllum commune. The A alpha locus contains two homeobox genes, Y and Z, encoding two homeodomain-related proteins, Y and Z. Y and Z are each multiallelic genes. When haploid strains form fusion cells, only particular combinations of Y and Z alleles activate A alpha-regulated sexual development. The role of the putative homeodomain was examined in several Y and Z alleles by site-directed mutagenesis of regions critical to secondary structure and function of homeodomains. Mutations of the Z homeobox do not affect the function of Z proteins in A alpha-activated development, but mutations of Y homeoboxes destroy the ability of Y proteins to activate development. We conclude that only one of two A alpha homeodomain-related regulators relies upon the homeodomain motif to effect gene expression in sexual development. This conclusion affords a refinement of our working hypothesis for the mechanism by which A alpha proteins may regulate target gene expression. On the basis of our results with the Z protein, we speculate that the DNA-binding motifs of some transcriptional regulators may be lost or modified during evolution once these regulators have been recruited to participate in complexes with other DNA-binding proteins.