Excision of lambda prophage: Effects on host survival

Induction with ultraviolet light of K12 lysogens carrying defective prophages leads to death of the host cell. In order to determine whether the killing accompanying induction was due to diffusible phage products, or was a result of prophage excision itself, the survival of lysogens carrying DNA defective prophages on an extra chromosomal site (the F-gal episome), were examined. Induction of prophages, carrying mutations in the x region, or the N, O or P cistrons, from this location, did not result in host death, but, with the exception of mutants in the x region of the lambda chromosome, it did result in loss of the episome. On the basis of these results, it is proposed that the lethal event in induction of the DNA-defective mutants is due to prophage excision, which leads to interruption of the host chromosome, and cell death.