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Listeria monocytogenesis well known for its durable physiological characteristics, which allow the organism to grow at low temperature and pH and high osmolarity. Growth under high osmolarity depends on the accumulation of compatible solutes, among which glycine betaine and carnitine are the preferred solutes for this organism. Three different transport systems, Gbu, BetL, and OpuC, have been identified inL. monocytogeneswhich serve to scavenge the preferred compatible solutes. The general stress response regulator σBhas been shown to play an important role in osmotic adaptation inL. monocytogenes, presumably by directing transcription from one or more of the solute transport genes. In the studies presented here, we have used primer extension analyses to identify the promoter elements responsible for transcription of theopuC,gbuA, andbetLgenes. All three genes are osmotically inducible to some degree.betLis transcribed from a σB-independent promoter, whilegbuAis transcribed from dual promoters, one of which is σBdependent.opuCis transcribed exclusively from a σB-dependent promoter. ThebetLpromoter is similar in sequence to the σB-independentgbuAP1 promoter. Kinetic analysis of transcript accumulation after osmotic upshift demonstrated that σB-dependent transcripts fromgbuAP2 andsigBaccumulate for an extended period after upshift, suggesting that σBactivity may provide a mechanism for sustained high-level expression during osmotic challenge. In contrast to osmotic upshift, expression from the σB-dependentopuCandgbuAP2 promoters after temperature upshift and ethanol stress was minimal, suggesting that additional mechanisms may also participate in regulating transcription from these σB-dependent promoters.

Listeria monocytogenes is well known for its robust physiology, which permits growth at low temperatures under conditions of high osmolarity and low pH. Although studies have provided insight into the mechanisms used by L. monocytogenes to allay the physiological consequences of these adverse environments, little is known about how these responses are coordinated. In the studies presented here, we have cloned the sigB gene and several rsb genes from L. monocytogenes , encoding homologs of the alternative sigma factor ς B and the RsbUVWX proteins, which govern transcription of a general stress regulon in the related bacterium Bacillus subtilis . The L. monocytogenes and B. subtilis sigB and rsb genes are similar in sequence and physical organization; however, we observed that the activity of ς B in L. monocytogenes was uniquely responsive to osmotic upshifting, temperature downshifting, and the presence of EDTA in the growth medium. The magnitude of the response was greatest after an osmotic upshift, suggesting a role for ς B in coordinating osmotic responses in L. monocytogenes . A null mutation in the sigB gene led to substantial defects in the ability of L. monocytogenes to use betaine and carnitine as osmoprotectants. Subsequent measurements of betaine transport confirmed that the absence of ς B reduced the ability of the cells to accumulate betaine. Thus, ς B coordinates responses to a variety of physical and chemical signals, and its function facilitates the growth of L. monocytogenes under conditions of high osmotic strength.