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Abstract

The filamentous fungus Neurospora crassa has two mating-types, A and a, responsible for mating and vegetative heterokaryon incompatibility. The mt locus is composed of dissimilar DNA sequences termed idiomorphs. In a individuals, mt a-1 is the gene responsible for regulation of all sexual and vegetative functions of the mating-type locus. In A strains, mt A-1 is responsible for fertilisation and heterokaryon incompatibility functions. Two additional genes of the A idiomorph, mt A-2 and mt A-3, were isolated and characterised in this study. The mt A-2 and mt A-3 genes code for proteins which have characteristics of transcription factors. Isolation of mt A-2 and mt A-3-specific mutants indicated that mutations at these genes lead to arrest of the sexual development at about 3 days after fertilisation, but only if both mt A-2 and mt A-3 are affected. Six different mating-type mutants defective at mt A-1, mt A-2, mt A-3 and mt a-1 were used in transcriptional analyses of genes preferentially expressed during early sexual development (sdv genes). The results obtained with these analyses suggested the involvement of all four mating-type proteins in regulation of some of these genes. Regulation of different genes by N. crassa mating-type seems to depend on combinatorial interactions between the different mating-type proteins.