Mechanisms that Determine the Differential Stability of Stx(+) and Stx(−) Lysogens

Phages 933W, BAA2326, 434, and λ are evolutionarily-related temperate lambdoid phages that infect Escherichia coli. Although these are highly-similar phages, BAA2326 and 933W naturally encode Shiga toxin 2 (Stx(+)), but phage 434 and λ do not (Stx(−)). Previous reports suggest that the 933W Stx(+) prophage forms less stable lysogens in E. coli than does the Stx(−) prophages λ, P22, and 434. The higher spontaneous induction frequency of the Stx(+) prophage may be correlated with both virulence and dispersion of the Stx2-encoding phage. Here, we examined the hypothesis that lysogen instability is a common feature of Stx(+) prophages. We found in both the absence and presence of prophage inducers (DNA damaging agents, salts), the Stx(+) prophages induce at higher frequencies than do Stx(−) prophages. The observed instability of Stx(+) prophages does not appear to be the result of any differences in phage development properties between Stx(+) and Stx(−) phages. Our results indicate that differential stability of Stx(+) and Stx(−) prophages results from both RecA-dependent and RecA-independent effects on the intracellular concentration of the respective cI repressors.