Rac-E. coli K12 strains carry a preferential attachment site for lambda rev.

Lambda rev is a hybrid lambdoid phage formed by recombination between lambda and a defective lambdoid prophage (Rac) present in most E. coli K12 derivatives. We show here that three independently derived Rac-E. coli K12 strains are specifically deleted for the entire Rac prophage consistent with loss of Rac by excisive recombination between hybrid attachment sites that flank the prophage (c.f. excision of a lambda prophage). lambda rev, in which int and PP' of lambda have been replaced by integrative recombination genes and an attachment site derived from Rac (Gottesman et al. 1974), integrates site-specifically and in the correct orientation at the preferential attachment site generated by Rac excision.