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RsbT and RsbV Contribute to ^B-Dependent Survival under Environmental, Energy, and Intracellular Stress Conditions in Listeria monocytogenes

Department of Food Science, Cornell University, Ithaca, New York

Received 14 January 2004/ Accepted 17 May 2004

Sigma B (^B) is a stress-responsive alternative sigma factor that has been identified in various gram-positive bacteria. Seven different regulators of sigma B (Rsbs) are located in the sigB operons of both Bacillus subtilis and Listeria monocytogenes. In B. subtilis, these proteins contribute to regulation of ^B activity by conveying environmental and energy stress signals through two well-established branches of a signal transduction pathway. RsbT contributes to regulation of ^B activity in response to environmental stresses, while RsbV contributes to ^B activation under both environmental and energy stresses in B. subtilis. To probe L. monocytogenes Rsb roles in ^B-mediated responses to various stresses, in-frame deletions were created in rsbT and rsbV. Phenotypic characterization of the L. monocytogenes rsbT and rsbV null mutants revealed that both mutants were similar to the sigB strain in their abilities to survive under environmental stress conditions (exposure to synthetic gastric fluid, pH 2.5, acidified brain heart infusion broth [BHI], or oxidative stress [13 mM cumene hydroperoxide]). Under energy stress conditions (carbon starvation in defined media, entry into stationary phase, or reduced intracellular ATP), both rsbT and rsbV showed survival reductions similar to that of the sigB strain. These observations suggest that the pathways for Rsb-dependent regulation of ^B activity differ between L. monocytogenes and B. subtilis. As ^B also activates transcription of the L. monocytogenes prfAP2 promoter, we evaluated virulence-associated characteristics of prfAP1rsbT and prfAP1rsbV double mutants in hemolysis and tissue culture assays. Both double mutants showed identical phenotypes to prfAP1P2 and prfAP1sigB double mutants, i.e., reduced hemolysis activity and reduced plaque size in mouse fibroblast cells. These findings indicate that RsbT and RsbV both contribute to ^B activation in L. monocytogenes during exposure to environmental and energy stresses as well as during tissue culture infection.

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