Starvation-associated mutation in Escherichia coli: a spontaneous lesion hypothesis for "directed" mutation

When stationary phase E. coli WU3610, carrying an ochre mutation in the tyrA gene, were incubated on plates lacking tyrosine, tyrosine-independent (Tyr+) mutants appeared from day 7 onwards in a time-dependent manner. These starvation-associated mutants did not contain either identifiable tRNA suppressors or reversions at the ochre site and are thus quite distinct from the mutants commonly found to arise during active growth. When an appropriate fluctuation assay protocol was employed slow growing Tyr+ mutants were also found to arise in growing cells, and their distribution was more characteristic of a replication-dependent than a time-dependent process. The rate of appearance of starvation-associated mutants at 37 degrees C was somewhat less than at 27 degrees C and this was attributed to a reduction in viability at the higher temperature. There was no evidence for the accumulation on the plates of mutations in other genes. Tyr+ mutants were, however, shown to arise during incubation of stationary phase cells under conditions where there was no selection for tyrosine independence, provided outgrowth was subsequently permitted on plates lacking tyrosine. This distinguishes the present system from those systems exhibiting genuine "directed" or "adaptive" mutation, should they exist. To explain the specificity which occurs, it is proposed that the appearance of stationary mutants in ochre strains reflects the time-dependent accumulation in the transcribed strand of a DNA lesion that has a high probability of miscoding during transcription and replication. A mutant RNA transcript permits protein synthesis which in turn triggers DNA replication. The mutation is then fixed in the DNA as a permanent heritable change. The apparent "directedness" of the process is, on this model, determined solely by the particular miscoding DNA lesion occurring in a transcribed strand at a site where a change in phenotype permits DNA replication to occur.