THE VSR ENDONUCLEASE OF ESCHERICHIA-COLI - AN EFFICIENT DNA-REPAIR ENZYME AND A POTENT MUTAGEN

The Vsr endonuclease of Escherichia coli initiates the repair of T/G m ismatches caused by deamination of 5-methylcytosine to thymine, In thi s paper, we examine the capacity of Vsr to prevent CG-to-TA mutations in cells with increased transcription of the cytosine methylase gene ( dcm). We find that sufficient Vsr is produced by a single chromosomal copy of vsr to prevent mutagenesis. We also investigate the cause of t he transition and frameshift mutations in cells overproducing Vsr, Nei ther the absence of the dcm methylase nor its overproduction affects V sr-stimulated mutagenesis, However, addition of mutS, mutL, or mutH on multicopy plasmids has a significant effect: mutL or mutH decreases t he number of mutations, while mutS stimulates mutagenesis. The mut-con taining plasmids have the same effect in cells treated with 2-aminopur ine and in cells made defective in DNA proofreading, two experimental situations known to cause transition and frameshift mutations by satur ating mismatch repair.