Abstract
λqsr′ (Q-independent) phages are characterised by the replacement of the region of the λ 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 λp4 and λqin A 3, two such Q-independent phages, are the product of recombination between λ 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 K 12 strain AB1157 from which λqsr′ 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.