EXPRESSION OF THE ESCHERICHIA-COLI ADA GENE IN SACCHAROMYCES-CEREVISIAE PROVIDES CELLULAR-RESISTANCE TO N-METHYL-N'-NITRO-N-NITROSOGUANIDINE IN RAD6 BUT NOT IN RAD52 MUTANTS

The Escherichia coli ada gene protein coding region under the control of the yeast alcohol dehydrogenase promoter in the extrachromosomally replicating yeast expression vectors pADHO6C and pVT103LO6C was introd uced into the wild-type yeast strains, YNN-27 and FF-18733, and the re pair deficient mutants LN-1 (rad1-1), VV-5 (rad6-1), C5-6 (rad52-1) an d FF-18742 (rad52::URA3), This resulted in the expression of 3950, 190 0, 1870, 1620, 1320 and 1420 fmol ada-encoded ATase/mg protein respect ively: transformation with the parent vectors resulted in ATase activi ties of 3-17 fmol/mg protein. The wild-types, rad1-1 and rad6-1 yeast expressing the bacterial ATase showed increased resistance to the toxi c and mutagenic effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) . Expression of ATase in the rad52-1 and rad52::URA3 mutants neither c omplemented their sensitivity, nor reduced the mutagenic effects of th is agent. These results suggest that whilst a portion of the toxic and mutagenic lesions induced by MNNG can be repaired in yeast by the E.c oli Ada protein in a RAD1- and RAD6-independent manner, the RAD52 gene product may be essential for the complete functioning of the Ada ATas e. This is the first suggestion of a possible cofactor requirement for ATase.