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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 x 10(-8), 4.13 x 10(-8) and 5.00 x 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 4,959 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 lambda 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.

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