Effect of Salt Shock on Stability of λ ^imm434 Lysogens

Abstract

The affinities of the bacteriophage 434 repressor for its various binding sites depend on the type and/or concentration of monovalent cations. The ability of bacteriophage 434 repressor to govern the lysis-lysogeny decision depends on the DNA binding activities of the phage's cI repressor protein. We wished to determine whether changes in the intracellular ionic environment influence the lysis-lysogeny decision of the bacteriophage imm434 . Our findings show that the ionic composition within bacterial cells varies with the cation concentration in the growth media. When imm434 lysogens were grown to mid-log or stationary phase and subsequently incubated in media with increasing monovalent salt concentrations, we observed a salt concentration-dependent increase in the frequency of bacteriophage spontaneous induction. We also found that the frequency of spontaneous induction varied with the type of monovalent cation in the medium. The saltdependent increase in phage production was unaffected by a recA mutation. These findings indicate that the salt-dependent increase in phage production is not caused by activation of the SOS pathway. Instead, our evidence suggests that salt stress induces this lysogenic bacteriophage by interfering with 434 repressor-DNA interactions. We speculate that the salt-dependent increase in spontaneous induction is due to a direct effect on the repressor's affinity for DNA. Regardless of the precise mechanism, our findings demonstrate that salt stress can regulate the phage lysis-lysogeny switch.In vitro studies show that the stability and specificity of protein-DNA complexes are remarkably dependent on the type and concentration of ions present in the solvent milieu (for a review, see reference 43). In large measure, the sensitivities of these complexes to changes in the salt concentration derive from the contributions of charge-charge interactions between the protein and DNA. In addition, the binding of cations to DNA and/or protein-DNA complexes can influence complex stability by modulating the overall structure of the protein-DNA interface (4,5,32). The demonstrated importance of ionic conditions for protein-DNA complex formation in vitro suggests that the intracellular ionic environment may also influence protein-DNA interactions in vivo.We were interested in knowing whether changes in the intracellular ionic environment influence the lysis-lysogeny decision of lambdoid bacteriophages, in particular, bacteriophage 434 that infects Escherichia coli. Similar to bacteriophage , imm434 is a temperate phage whose life cycle alternates between the lysogenic and lytic developmental pathways (11,38). In a lysogen, the phage's genome is integrated into the chromosome of its host and is replicated along with the host chromosome. The lysogen is a metastable developmental state; all lysogenized phage can undergo lytic development (40). In lytic growth, phage DNA is not integrated into the chromosome; rather, its intracellular replication, assembly into phage particles, and subsequent host cell ly...

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