Very-short-patch repair in Escherichia coli requires the dam adenine methylase

Strains of Escherichia coli which lack the dam-encoded adenine methylase ar e mutators due to a reduction in the efficiency of postreplication mismatch repair, In this study, we show that Dam(-) strains are also defective in v ery-short-patch repair, the system which corrects TIG mismatches arising fr om the deamination of 5-methylcytosine, This defect is associated with decr eased levels of Vsr, the endonuclease which initiates short-patch repair. W e also show that production of the dcm-encoded cytosine methylase is unaffe cted in Dam(-) strains. Since the dcm and vsr genes are cotranscribed, the regulation of Vsr by Dam is probably posttranscriptional.