Role of the RuvAB protein in avoiding spontaneous formation of deletion mutations in the Escherichia coli K-12 endogenous tonB gene

The endogenous tonB gene of Escherichia coli was used as a target for spontaneous deletion mutations which were isolated from ruvAB ^-, recG ^-, and ruvC ^- cells. The rates of tonB mutation were essentially the same in ruv ⁺, ruvAB ^-, recG ^-, and ruvC ^- cells. We analyzed tonB mutants by sequencing. In the ruv ⁺, recG ^-, and ruvC ^- strains, the spectra were different from those obtained from the ruvAB ^- cells, where deletions dominated followed by IS insertions, base substitutions, and frameshifts, in that order. We then analyzed the tonB-trp large deletion, due to simultaneous mutations of the trp operon, and found that the frequency in ruvAB ^- was higher than those in ruv ⁺, recG ^-, and ruvC ^- cells. To characterize deletion formation further, we analyzed all the tonB mutants from one colicin plate. Seven deletions were identified at five sites from the 45 tonB mutants of ruv ⁺ cells and 24 deletions at 11 sites from the 43 tonB mutants of ruvAB ^- cells. Thus, the ruvAB ^- strain is a deletion mutator. We discuss the role of RuvAB in avoiding deletions.