Host survival following infection with or induction of bacteriophage lambda mutants.

The lethal effects of lambda infection and lambda induction were studied using conditional lethal mutants of phage lambda in the su- host W3350. Phage with mutations in Q and R, which are blocked after DNA replication, kill promptly and efficiently on induction or infection. Infection with mutants defective in O and P, which do not replicate, kills less efficiently, although these mutants do kill effectively at high multiplicity of infection. Heat induction of lysogens carrying DNA defective mutants O or P promptly arrests host DNA synthesis, but this leads to killing only after a considerable lag. Heating also blocks transfer of an F-gal episome by a lysogen in which a temperature inducible O or P mutant is carried on the episome. These effects of heating are reversed by cooling, which leads to recovery of host DNA synthesis and recovery of the ability to transfer the episome. The effects of heating lysogens for temperature inducible DNA defective phage are interpreted to result from interruption of the Escherichia coli chromosome (or episome), which in itself, is not lethal to the host. Their reversal on cooling is attributed to repair of the break in the chromosome, which permits survival, often associated with curing. Evidence is presented that interruption of the E. coli chromosome can also be produced by infection, but the relationship of this event to loss of viability on infection is still uncertain. N mutants kill even less efficiently than O and P mutants on infection and show killing and DNA arrest on thermal induction only after an initial stimulation of DNA synthesis. They neither cure appreciably, nor resume DNA synthesis on cooling, once DNA synthesis is arrested. Not all the properties of N mutants can presently be explained.