RecA-independent pathways of lambdoid prophage induction in Escherichia coli

Two Escherichia coli genes, expressed from multicopy plasmids, are shown to cause partial induction: of prophage λ in recA mutant lysogens. One is rcsA, which specifies a positive transcriptional regulator of the cps genes, which are involved in capsular polysaccharide synthesis. The other is dsrA, which specifies an 85-nucleotide RNA that relieves repression of the rcsA gene by histone-like protein H-NS. Genetic contexts known to increase Cps expression also cause RecA-independent λ induction: the rcsC137 mutation, which causes constitutive Cps expression, and the Ion and rcsA3 mutations, which stabilize RcsA. Lambdoid phages 21, φ80, and 434 are also induced by RcsA and DsrA overexpression in recA lysogens. Excess λ cI repressor specifically blocks λ induction, suggesting that induction involves repressor inactivation rather than repressor bypass. RcsA-mediated induction requires RcsB, the known effector of the cps operon, whereas DsrA-mediated induction is RcsB independent in stationary phase, pointing to the existence of yet another RecA-independent pathway of prophage induction.