MMS2, ENCODING A UBIQUITIN-CONJUGATING-ENZYME-LIKE PROTEIN, IS A MEMBER OF THE YEAST ERROR-FREE POSTREPLICATION REPAIR PATHWAY

Among the three Saccharomyces cerevisiae DNA repair epistasis groups, the RAD6 group is the most complicated and least characterized, primar ily because it consists of two separate repair pathways: an error-free postreplication repair pathway, and a mutagenesis pathway, The rad6 a nd rad18 mutants are defective in both pathways, and the rev3 mutant a ffects only the mutagenesis pathway, but a yeast gene that is involved only in error-free postreplication repair has not been reported. We c loned the MMS2 gene from a yeast genomic library by functional complem entation of the mms2-1 mutant [Prakash, L. & Prakash, S. (1977) Geneti cs 86, 33-55]. MMS2 encodes a 137-amino acid, 15.2-kDa protein with si gnificant sequence homology to a conserved family of ubiquitin-conjuga ting (Ubc) proteins, However, Mms2 does not appear to possess Ubc acti vity. Genetic analyses indicate that the mms2 mutation is hypostatic t o rad6 and rad18 but is synergistic with the rev3 mutation, and the mm s2 mutant is proficient in UV-induced mutagenesis. These phenotypes ar e reminiscent of a pol30-46 mutant known to be impaired in postreplica tion repair, The mms2 mutant also displayed a REV3-dependent mutator p henotype, strongly suggesting that the MMS2 gene functions in the erro r-free postreplication repair pathway, parallel to the REV3 mutagenesi s pathway, Furthermore, with respect to UV sensitivity, mms2 was found to be hypostatic to the rad6(Delta 1-9) mutation, which results in th e absence of the first nine amino acids of Rad6, On the basis of these collective results, we propose that the mms2 null mutation and two ot her allele-specific mutations, rad6(Delta 1-9) and pol30-46, define th e error-free mode of DNA postreplication repair, and that these mutati ons may enhance both spontaneous and DNA damage-induced mutagenesis.