Functional analyses of Proteus mirabilis wild-type and mutant RecBCD enzymes in Escherichia coli reveal a new mutant phenotype.

Cloned into Escherichia coli the recB, recC and recD genes of Proteus mirabilis produce a recBCD enzyme (exoV) functional in recombination and DNA repair. The direction of transcription of recB, recC and recD, the sizes of the enzyme subunits, and their composition in the active enzyme are similar to that observed for the E. coli enzyme. In lambda crosses, the P. mirabilis enzyme has only about 40% of the Chi activity of the E. coli enzyme. The recBCD genes were also cloned from an exoV mutant of P. mirabilis which is u.v.-sensitive and partly deficient in exoV. The defect was attributed to the recB gene by complementation studies. In a recBCD deletion strain of E. coli, the enzyme from the mutant produced 40% of conjugational recombinants and had retained about 25% of Chi activity. However, it did not restore normal DNA repair, cell viability or recombination in lambda crosses and P1 transduction. The new mutant phenotype is discussed in the light of the assumption that prokaryotic recBCD enzymes can promote recombination in a Chi-dependent and a Chi-independent manner.