THE MUTATOR GENE SWI8 EFFECTS SPECIFIC MUTATIONS IN THE MATING-TYPE REGION OF SCHIZOSACCHAROMYCES-POMBE

The swi8(+) gene of Schizosaccharomyces pombe appears to be involved i n the termination step of copy synthesis during mating-type (MT) switc hing. Mutations in swi8 confer a general mutator phenotype and, in par ticular, generate specific mutations in the MT region. Sequencing of t he MT cassettes of the h(90) swi8-137 mutant revealed three altered si tes. One is situated at the switching (smt) signal adjacent to the Hf homology box of the expression locus mat1:1. It reduces the rate of MT switching. The alteration at the smt signal arose frequently in other h(90) swi8 strains and is probably caused by gene conversion in which the sequence adjacent to the H1 box of mat2:2 is used as template. Th is change might be generated during the process of MT switching when h ybrid DNA formation is anomalously extended into the more heterologous region flanking the H1 homology box. In addition to the gene conversi on at mat1:1, two mutations were found in the H3 homology boxes of the silent cassettes mat2:2 and mat3:3.