Mutagenic specificity of ultraviolet light in the tonB gene on the chromosome of Escherichia coli uvrA cells.

We have analyzed the DNA sequence changes in a total of 60 ultraviolet-induced mutations in the endogenous tonB gene of Escherichia coli uvrA strain. Of the mutations 82% were base substitutions among which G:C-->A:T transition predominated. Three GG-->AA tandem double-base substitutions, which are thought to originate from UV damage, were also observed. The sites where base substitutions occurred were correlated with sequences of adjacent pyrimidines, indicating mutation-targeted UV photoproducts. G:C-->A:T transition in the tonB gene mutation can be exclusively observed at either the 3' side of the TC site which is on the template for the lagging strand of DNA replication or the 5'/3' sides of the CC site on the template for the leading strand. We hypothesize that this extreme strand specificity is due to a difference in fidelity of DNA replication of the leading and the lagging strand.