HYPERSPECKLED MUTANTS OF SCHIZOSACCHAROMYCES-POMBE - FREQUENT MATING-TYPE SWITCHING WITHOUT DETECTABLE DOUBLE-STRAND BREAKS

Mating-type (MT) switching in homothallic (h(90)) strains of Schizosac haromyces pombe is initiated by a DNA double-strand break (DSB) at the distal end of the expression cassette mat1. The cis-acting smt-s1 mut ation C13-P11 reduces the frequency of MT switching. It is a small del etion mapping approximately 50 bp distal to the site of the DSB. From the h(90) smt-s1 strain we isolated 13 mutants with a hyperspeckled io dine reaction. In these mutants the frequency of MT switching is incre ased. The mutations define nine different hsp genes, none of which map s in or close to the MT region. We tested one mutant of each gene for the presence of DSBs at mat1. Curiously, in none of the h(90) smt-s1 h sp strains could DSBs be detected, although some sporulate nearly as e fficiently as the h(90) smt-n wild type. The hsp mutations show no eff ect in smt-0 strains; the smt-0 deletion abolishes MT switching comple tely. Furthermore, we tested the interaction of hsp1-1 with swi1, swi2 and swi7 mutations. hsp1-1 has no effect in swi2 strains, whereas it increases MT switching in swi7 and, to a lesser degree, in swi1 mutant s.