Biochemical and genetic studies of recombination proficiency in Escherichia coli. II. Rec+ revertants caused by indirect suppression of rec- mutations.

All recB(-) and recC(-) mutants of E. coli carry out significant residual genetic recombination, whereas all recA(-) mutants form no recombinants. This observation suggests that an alternative minor pathway of recombination, independent of recB(+) and recC(+) products, may be operative in Escherichia coli. Rec(+) revertants of recB(-)recC(+), recB(+)recC(-), and recB(-) strains of E. coli have been isolated and are shown to fall into at least two major genotypic classes. One class carries revertant mutations which map in or very near the recB and recC genes. In this class an ATP-dependent DNase characteristic of wild type E. coli is restored. The reversions in this class are probably back-mutations or intragenic suppressor mutations. A second class carries revertant mutations which are located far from the recB and recC genes. In this class there is a high level of DNase activity which does not require ATP and is inactive on T4 DNA. Indirect and not informational suppression appears to be responsible for the second class of revertants. The suggestion is made that restoration of recombination by indirect suppression involves an activation or derepression of one or a series of enzymes, which participate in a pathway of recombination, alternative to the recB and recC pathway, but normally of minor importance. The ATP-independent DNase may be one of these enzymes.