The UV excision-repair system of Saccharomyces cerevisiae is involved in the removal of methylcytosines formed in vivo by a cloned prokaryotic DNA methyltransferase.

DNA methyltransferase activity is not normally found in yeast. To investigate the response of Saccharomyces cerevisiae to the presence of methylated bases, we introduced the Bacillus subtilis SPR phage DNA-[cytosine-5] methyltransferase gene on the shuttle vector, YEp51. The methyltransferase gene was functionally expressed in yeast under the control of the inducible yeast GAL 10 promoter. Following induction we observed a time-dependent methylation of yeast DNA in RAD+ and rad2 mutant strains; the rad2 mutant is defective in excision-repair of UV-induced DNA damage. Analysis of restriction endonuclease digestion patterns revealed that the relative amount of methylated DNA was greater in the excision defective rad2 mutant than in the RAD+ strain. These data indicate that the yeast excision-repair system is capable of recognizing and removing m5C residues.