The origin of Q-independent derivatives of phage lambda.

lambda qsr' (Q-independent) phages are characterised by the replacement of the region of the lambda genome that contains Q, S, R, and the late gene promoter, P'R, with host-derived DNA that codes for functions analogous to those deleted. Restriction endonuclease analysis and DNA/DNA hybridisation methods have been used to show that lambda p4 and lambda qin A3, two such Q-independent phages, are the product of recombination between lambda and a defective lambdoid prophage (the qsr' prophage) located at an as yet unidentified site in the E. coli K 12 chromosome. The qsr' prophage is distinct from the defective lambdoid prophage Rac (Kaiser and Murray 1979). In the E. coli K12 strain AB1157 from which lambda qsr' phages cannot be generated, the qsr' prophage has suffered an internal deletion. That the qsr' prophage appears not to carry a full complement of essential late genes suggests one explanation for its apparently defective nature.