Frameshifts, base substitutions and minute deletions constitute X-ray-induced mutations in the endogenous tonB gene of Escherichia coli K12.

We have analyzed the DNA sequence changes in a total of 127 X-ray-induced mutations in the endogenous tonB gene of Escherichia coli cells. Frameshifts accounted for 61 mutations among which 51 were a - 1 frameshift. The second most commonly found mutations were base substitutions (20 transversions and 8 transitions). Twelve of the 16 deletion mutations were the minute-size deletion of 3-25 base pairs, three were the medium-size deletion of 294-643 base pairs and the remaining one was the deletion of 8375 base pairs. Half of the frameshifts and deletions had a run of several identical bases or short direct repeats at the sites of mutation. The spectrum was not in good agreement with the spectrum of spontaneous endogenous tonB mutation nor with the spectra obtained from a mutated gene on a plasmid which had been irradiated in vitro and used to transfect cells for the assay. We discuss the possibility that an X-ray-induced DNA strand break produces local alteration of DNA structure which increases aberrant DNA replication leading to frameshift and minute-size deletion mutations.