A CHIMERIC HOMEODOMAIN PROTEIN CAUSES SELF COMPATIBILITY AND CONSTITUTIVE SEXUAL DEVELOPMENT IN THE MUSHROOM COPRINUS-CINEREUS

The A mating type genes of the mushroom Coprinus cinereus encode two c lasses of putative transcription factor with distinctive homeodomain m otifs (HD1 and HD2). A successful mating brings together different all elic forms of these genes and this triggers part of a developmental se quence required for sexual reproduction. In this report we provide evi dence that this developmental programme is promoted by a physical inte raction between the two classes of homeodomain protein. Rare dominant mutations conferring self-compatibility map to the A locus and result in constitutive operation of the A-regulated developmental pathway. Ou r molecular analysis of one of these mutations shows that it has gener ated a chimeric gene by in-frame fusion of an HD2 and an HD1 gene. Fus ion has overcome the normal incompatibility between two proteins coded by genes of the same A locus and generated a protein that is sufficie nt to promote development in the absence of any other active A mating type genes. The fusion protein retains most of the HD2 sequence, but o nly the C-terminal part of the HD1 protein. It has only the HD2 homeod omain motif as a potential DNA binding domain fused to an essential C- terminal region of the HD1 protein, which in a normal HD1-HD2 protein complex may be the major activation domain.