A method for the isolation of phage mutants altered in their response ot lysogenic induction.

Phage lambdacl+ gives clear plaques whereas phage lambdacIind- gives turbid plaques on a lawn of a mutant strain of E. coli K12. This strain, called STS, carries mutation spr in a tif sfi genetic background. I hypothesize that upon temperate phage infection, STS bacteria spontaneously inactivate phage repressor by the same mechanism involved in normal lysogenic induction which results in obligatory lytic growth of lambda+. The use of the STS mutant facilitates the isolation and genetic analysis of phage mutants with an abnormal response to lysogenic induction.