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. (C) 2004 Elsevier Inc. All rights reserved.