Roles of the recJ and recQ proteins in spontaneous formation of deletion mutations in the Escherichia coli K12 endogenous tonB gene

The endogenous tonB gene of Escherichia coli was used as a target for spontaneous deletion mutations which were isolated from recJ^- and recQ^- cells. Large deletions, due to simultaneous mutations of the trp operon, were also isolated. The rates of tonB mutation were 2.77 × 10^-8, 4.13 × 10^-8 and 5.00 × 10^-8 for rec⁺, recJ^- and recQ^- cells, respectively. We analyzed 94 and 99 tonB mutants from the recJ^- and recQ^- cells, respectively, by sequencing. We found that IS insertion dominated, followed by base substitutions, frameshifts and deletions in both recJ^- and recQ^- strains. We then analyzed 55 tonB-trp deletions, ranging in size from 5907 to 20 832 bp, from the recJ^- strains and 47 tonB-trp deletions, ranging in size from 4959 to 16 390 bp from the recQ^- strains. About one-third of tonB-trp deletions from both the recJ^- and the recQ^- cells were found to have occurred between short sequence repeats at the deletion termini. About one-third of tonB-trp deletions from both mutants showed 2-4 bp repeats in the immediate vicinity of the endpoints, which appeared to indicate no clear association with deletion. The remaining one-third of tonB-trp deletions had no homology at the endpoint. These results were similar to those for the rec⁺ cells. Hanada and colleagues demonstrated that structually similar rearrangements arising during λ bio phage formation (illegitimate recombination) increased in the recQ^- strain. To explain this discrepancy, we interpreted as distinctive the mechanism for rearrangement during transducing phage formation which is recQ-dependent and that for deletions formed in chromosomes which is recQ-independent.