Induction of prophage lambda during the division cycle of Escherichia coli.

When synchronous populations of Escherichia coli B/r (lambda) were exposed to low doses of ultraviolet light, the yield of infective centres varied with cell age. The yield was highest if the lysogenic bacteria were irradiated at a time which coincides approximately with the termination of rounds of DNA replication and it was lowest when dividing cells were irradiated. No such variation was detected following either irradiation of excision-defective lysogenic cells or thermal induction of lambdacI857 prophage in irradiated bacteria. It is suggested that the variation reflects a relationship between prophage induction and inhibition of cell division. This hypothesis is supported by data showing that irradiation promoted induction and curtailed division in E. coli K12 dnaA mutants which were dividing in the absence of DNA replication.