Two enzymes, both of which process recombination intermediates, have opposite effects on adaptive mutation in Escherichia coli.

Reversion of a lac- frameshift allele carried on an F' episome in Escherichia coli occurs at a high rate when the cells are placed under lactose selection. Unlike Lac+ mutations that arise during nonselective growth, the production of these adaptive mutations requires the RecA-RecBCD pathway for recombination. In this report, we show that enzymes that process recombination intermediates are involved in the mutagenic process. RuvAB and RecG, E. coli's two enzymes for translocating Holliday junctions, have opposite effects: RuvAB is required for RecA-dependent adaptive mutations, whereas RecG inhibits them.