The Escherichla coll 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 Introduced Into the wild-type yeast strains, YNN-27 and FF-18733, and the repair deficient mutants LN-1 (rad1-1), W-5 (rad6-1), C5-6 (rad52-1) and FF-18742 (rad52::URA3). This resulted In the expression of 3950, 1900, 1870, 1620, 1320 and 1420 fmol acto-encoded ATase/mg protein respectively: transformation with the parent vectors resulted in ATase activities of 3-17 fmol/mg protein. The wild-types, radi-1 and rad6-1 yeast expressing the bacterial ATase showed Increased resistance to the toxic and mutagenic effects of N-methyi-W-nltro-W-nltrosoguanidlne (MNNG). Expression of ATase In the rad52-1 and rad52::URA3 mutants neither complemented their sensitivity, nor reduced the mutagenic effects of this 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.coli Ada protein in a RAD1- and AAD6-lndependent manner, the RAD52 gene product may be essential for the complete functioning of the Ada ATase. This Is the first suggestion of a possible cofactor requirement for ATase.