Sequences important for heterokaryon incompatibility function in MAT A-1 of Neurospora crassa

Strains of Neurospora crassa exist as two alternative mating type forms, A and a; differences in mating type are required for the initiation of the sexual cycle (Shiu and Glass, 2000). The mating-type (mat) locus also acts as a heterokaryon incompatibility (het) locus, such that hyphal fusion between A and a strains results in a heterokaryon that shows extremely inhibited growth, absence of conidiation, and hyphal compartmentation and death (Glass et al., 2000). The A and a mating type sequences occupy the same locus in A and a strains, but are highly dissimilar in sequence. The mat a-1 gene, which encodes a putative HMG (high mobility group) type of transcriptional regulator, provides all the functions for the a mating type, including mating, ascospore formation, and heterokaryon incompatibility (Chang and Staben, 1994). The mat A locus encodes three proteins. MAT A-2 and MAT A-3 are responsible for ascospore formation (Ferreira et al., 1998); MAT A-3 is a putative HMG type of transcriptional regulator. MAT A-1 is predicted to be a a-domain type of transcriptional regulator and is both necessary and sufficient to confer A mating specificity and trigger heterokaryon incompatibility with a strains (Glass et al., 1990). Mutations in an unlinked locus, tol, suppress mating-type incompatibility such that tol A and tol a strains are capable of forming a vigorous heterokaryon (Newmeyer, 1970; Shiu and Glass, 1999).