CLONING AND ANALYSIS OF THE MATING-TYPE GENES FROM COCHLIOBOLUS-HETEROSTROPHUS

Cochliobolus heterostrophus, a heterothallic Ascomycete, has a single mating type locus with two alternate forms called MAT-1 and MAT-2. MAT -1 was cloned by complementing a MAT-2 strain using a cosmid library f rom a MAT-1 strain and screening for a homothallic transformant. The c osmid recovered from this transformant was able to re-transform a MAT- 2 strain to homothallism and MAT identity was proven by restriction fr agment length polymorphism and conventional genetic mapping. All homot hallic transformants could mate with either MAT-1 or MAT-2 strains, al though the number of ascospores produced by self matings or crosses to MAT-2 strains was low. Progeny of selfed homothallic transformants we re themselves homothallic. MAT-2 was cloned by probing a cosmid librar y from a MAT-2 strain with a fragment of insert DNA from the MAT-1 cos mid. A 1.5 kb subclone of either MAT-containing cosmid was sufficient to confer mating function in transformants. Examination of the DNA seq uence of these subclones revealed that MAT-1 and MAT-2 contain 1297 bp and 1171 bp, respectively, of completely dissimilar DNA flanked by DN A common to both mating types. Putative introns were found (one in eac h MAT gene) which, when spliced out, would yield open reading frames ( ORFs) that occupied approximately 90% of the dissimilar DNA sequences. Translation of the MAT-1 ORF revealed similarity to the Neurospora cr assa MATA, Podospora anserina mat-, and Saccharomyces cerevisiae MATal pha1 proteins; translation of the MAT-2 ORF revealed similarity to the N. crassa MATa, P. anserina mat+, and Schizosaccharomycespombe mat-Mc proteins. These gene products are all proven or proposed DNA binding proteins. Those with similarity to MAT-2 are members of the high mobil ity group.