Mutagenesis in Escherichia coli : evidence for the mechanism of base change mutation by ultraviolet radiation in a strain deficient in excision-repair

Abstract

The mutagenic action of u. v. radiation has been studied upon Escherichia coli WP2 try her growing exponentially at 37 °C. Although this strain is unable to excise pyrimidine dimers from its DNA it showed no detectable reduction in growth rate after exposure to a dose of u. v. (10 -6 J mm -2 ) calculated to produce several dozen pyrimidine dimers per chromosome. As judged by photoreversibility of mutations to prototrophy, dimers at mutable sites may persist for up to about 4 generation times after u. v. and may give rise to mutations with a low probability in each replication cycle during this period. The slow disappearance of dimers takes place whether or not DNA replication is inhibited and indirect evidence suggests that excision-repair may not be involved. Mutations are established (i. e. become non-photoreversible) only when DNA replication is taking place and are not expressible on unsupplemented medium until approximately one generation time after being established. It is suggested that the mutation is initially produced by the laying down of an incorrect base opposite a pyrimidine dimer at, or shortly after, replication; the mutation becomes transcribable only after a further replication gives rise to a duplex mutant in both strands. The observed segregation pattern strongly suggests that at this further replication the mutation is carried by both resulting daughter duplexes. This implies that the mutant strand initially produced opposite the dimer has a strong influence on the bases of both new strands laid down at the next replication.