SUMMARY: The influence of post-irradiation growth inhibition on survival and absolute yields of prototrophic mutants has been studied in B/R, u.v.-irradiated in the lag and logarithmic phases of the growth cycle. Without post-irradiation growth inhibition, survival and yields of mutants were higher for logarithmic phase cells. Post-irradiation growth inhibition reduced yields of mutants from both types of cells to similar extents and markedly increased the survival of logarithmic phase cells, both after small and large u.v. doses. In the case of lag phase cells, a survival increase mediated by post-irradiation growth inhibition, could only be observed by giving a large u.v. dose or by inhibiting repair of damage occurring after plating for colony formation. Post-irradiation growth inhibition was observed to have two effects–one tending to increase survival and another tending to enhance lethality. The results indicate that observable effects of post-irradiation growth inhibition on survival and induced mutation in B/R and whether or not such effects are correlated depend upon a number of factors. These include: (1) relative amounts of repair occurring during growth inhibition and during subsequent growth; (2) heterogeneity of the population with respect to growth phase, intrinsic repair ability and amount of damage; and (3) relative contributions of increased repair and increased killing–both mediated by post-irradiation growth inhibition.


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