Comparative analysis of the mating-type loci from Neurospora crassa and Sordaria macrospora: identification of novel transcribed ORFs

The mating-type locus controls mating and sexual development in filamentous ascomycetes. In the heterothallic ascomycete Neurospora crassa, the genes that confer mating behavior comprise dissimilar DNA sequences (idiomorphs) in the mat a and mat A mating partners. In the homothallic fungus Sordaria macrospora, sequences corresponding to both idiomorphs are located contiguo usly in the mating-type locus. which contains one chimeric gene, Smt A-3, t hat includes sequences which are similar to sequences found at the matA and mat a mating-type idiomorphs in N. crassa. In this study. we describe the comparative transcriptional analysis of the chimeric mating-type region of S. macrospora and the corresponding region of the N. crassa mat a idiomorph . By means of RT-PCR experiments, we identified novel intervening sequences in the mating-type loci of both ascomycetes and, hence, concluded that an additional ORF, encoding a putative polypeptide of 79 amino acids, is prese nt in the N. crassa mat a idiomorph, Furthermore. our analysis revealed co- transcription of the novel gene with the mat a-1 gene in N. crassa. The sam e mode of transcription was found in the corresponding mating-type region o f S. macrospora where the chimeric Smt A-3 gene is co-transcribed with the mat rr-specific Smt a-1 gene. Analysis of a Sills A-3 cDNA revealed optiona l splicing of two introns. We believe that this is the first report of co-t ranscription of protein-encoding nuclear genes in filamentous fungi. Possib le functions of the novel ORFs in regulating mating-type gene expression ar e discussed.