Osmotolerance in Listeria monocytogenes : mechanisms and regulation of compatible solute accumulation
The work presented in this thesis describes the characterisation of the L-carnitine transporter, OpuC, belonging to the binding protein dependent ABC transporter superfamily. The transporter is encoded on a four gene operon, opuCABCD. The physiological study of two opuC mutants have revealed that this operon encodes the principal carnitine transport system in L. monocytogenes, and that the resulting transporter is specific for carnitine and not the related solute betaine. Usually the activity of this transporter is subject to negative regulation during growth in the presence of peptone. An opuCA deletion mutant retained the ability to utilise carnitine as an osmoprotectant at high concentrations (1 mM), and accumulated a cytoplasmic carnitine pool comparable to the wild-type, suggesting that a second low affinity carnitine transport system must exist in L. monocytogenes. Measurement of carnitine uptakes rates in the presence of 100 mM and 1 mM carnitine revealed that the rate of carnitine uptake in the DopuCA mutant was dependent on the carnitine concentration, confirming the low affinity of this unidentified system for carnitine. The stress inducible sigma factor, sB, is predicted to play a role in regulating the Listerial osmotic stress response. Studies utilising a sigB deletion mutant revealed that sB is required for the utilisation of carnitine as an osmoprotectant, by regulating the transcription of the opuC operon in response to hyperosmotic stress. Betaine accumulation is reduced in a strain lacking sB, in particular Na+ dependent betaine transport, although transcription of neither betaine transport systems, gbu and beiL, appear affected by the sB might play a post-transcriptional regulatory role in betaine accumulation.