Emergence of a mutagenic ochre suppressor mutation under lactose selectionin a ppm mutant of Escherichia coli harbouring the F ' lacZU118 episome

When Escherichia coli harbouring the ppm (earlier called adi) mutation and the F' lacZU118 episome is subjected to lactose selection in the presence o f suboptimal concentrations of glycerol, Lac(+) colonies emerge after 5-6 d ays. They are shown to harbour an ochre suppressor mutation at 15.15 min. I nactivation of recA results in approximately four-fold reduction in the res ponse. In the ppm - ochre suppressor double mutant background the leakiness of the lacZ allele carried by F' CC105 is enhanced, suggesting misreading of a valine codon (GUG) as glutamic acid codon (GAG). This is accompanied b y reversion of the lacZ mutation to lacZ(+) (GTG --> GAG). In LB medium bot h the leakiness and reversion are inhibited by streptomycin. Inactivation o f recA did not affect leakiness but abolished reversion. These data are dis cussed in relation to the importance of allele leakiness and restricted gro wth in stationary-phase (adaptive) mutagenesis.