Role of σ^B in regulating the compatible solute uptake systems of Listeria monocytogenes: Osmotic induction of opuC Is σ^B dependent

The regulation of the compatible solute transport systems in Listeria monocytogenes by the stress-inducible sigma factor σ^B was investigated. Using wild-type strain 10403S and an otherwise isogenic strain carrying an in-frame deletion in sigB, we have examined the role of σ^B in regulating the ability of cells to utilize betaine and carnitine during growth under conditions of hyperosmotic stress. Cells lacking σ^B were defective for the utilization of carnitine but retained the ability to utilize betaine as an osmoprotectant. When compatible solute transport studies were performed, the initial rates of uptake of both betaine and carnitine were found to be reduced in the sigB mutant; carnitine transport was almost abolished, whereas betaine transport was reduced to approximately 50% of that of the parent strain. Analysis of the cytoplasmic pools of compatible solutes during balanced growth revealed that both carnitine and betaine steady-state pools were reduced in the sigB mutant. Transcriptional reporter fusions to the opuC (which encodes an ABC carnitine transporter) and betL (which encodes an a secondary betaine transporter) operons were generated by using a promoterless copy of thegus gene from Escherichia coil Measurement of β-glucuronidase activities directed by opuC-gus and betL-gus revealed that transcription of opuC is largely σ^B dependent, consistent with the existence of a potential σ^B consensus promoter motif upstream from opuCA. The transcription of betL was found to be sigB independent. Reverse transcriptase PCR experiments confirmed these data and indicated that the transcription of all three known compatible solute uptake systems (opuC, betL, and gbu), as well as a gene that is predicted to encode a compatible solute transporter subunit (lmo421) is induced in response to elevated osmolarity. The osmotic induction of opuCA and lmo1421 was found to be strongly σ^B dependent. Together these observations suggest that σ^B plays a major role in the regulation of carnitine utilization by L. monocytogenes but is not essential for betaine utilization by this pathogen.