MUTAGENESIS OF YEAST MW104-1B STRAIN HAS IDENTIFIED THE UNCHARACTERIZED PMS6 DNA MISMATCH REPAIR GENE LOCUS AND ADDITIONAL ALLELES OF EXISTING PMS1, PMS2 AND MSH2 GENES

The haploid yeast Saccharomyces cerevisiae MW104-1B strain was disomic for chromosome III (n + 1) and carried DNA mismatches at three differ ent heteroallelic loci; leu2 (leu2-1/leu2-27), thr4 (thr4-1/thr4-16) a nd his4 (his4-4/his4-519) (Williamson, 1984). We mutagenized the MW104 -1B strain and identified seven mutant isolates that display elevated mitotic/meiotic prototrophs due to mismatch repair failures at heteroa llelic loci. Three mutants (pms1, pms2 and pms3) isolated earlier from MW104-1B were shown to correct in vitro constructed plasmids with def ined DNA mismatches (G/T, A/C, G/G, etc.) poorly (Kramer et al., 1989a ). Complementation tests were performed by crossing all seven new muta nt isolates to pms1 and pms2 mutants and assaying for mutant phenotype in the diploids. Four mutant isolates failed to complement the two kn own pms alleles (pms1-1 and pms2-1). Two other mutant isolates complem ented the pms1-1 and pms2-1 alleles, but failed to complement each oth er and were named as the pms5-1 allele of an uncharacterized gene (PMS 5). One other mutant isolate complemented the pms1-1, pms2-1 and pms5- 1 alleles and was named as the pms6-1 allele of another uncharacterize d gene (PMS6). Subsequently, the pms5-1 mutant allele was shown to be complemented by a plasmid borne yeast MSH2 gene, implying that it is a n allele: of MSH2 (PMS5). The human homologs (hMSH2 and hMLH1) of two yeast DNA mismatch repair genes (MSH2 and MLH1) have been cloned recen tly and shown to be responsible for hereditary nonpolypnosis colon can cer (HNPCC) (Fishel et al., 1993; Leach et al., 1993; Bronner et al., 1994; Papadopoulos et al., 1994).