The kinetics of derepression of prophage lambda following ultraviolet irradiation of lysogenic cells.

Double lysogens for prophages lambda cI+ and lambda cI ind-ts-857 are induced only by the combined effects of ultraviolet (UV) irradiation and high temperature, not by either treatment alone (Sussman and Jacob, 1962). We have followed the kinetics of inactivation of the cI+ repressor brought about by irradiation in asynchronously and synchronously growing cultures of B/r (lambda cI ind- ts-857). Assays of the yield of phage released as a result of temporary thermal inactivation of the UV-resistant ind- ts-857 repressor at intervals after the irradiation accurately reflect the time course of UV-induced inactivation of the cI+ repressor. The results show that UV-induced derepression takes place in all cells of the population approximately 20 min after the irradiation whether the cells were growing asynchronously or synchronously. Hence UV induction of prophage lambda is not triggered at a particular stage in the cell cycle.