kdpE and a putative RsbQ homologue contribute to growth of Listeria monocytogenes at high osmolarity and low temperature.

The kdp locus of Listeria monocytogenes encodes products with homology to structural proteins of a high-affinity potassium uptake system and to a two-component signal transduction system commonly involved in controlling gene expression. We have investigated the role of kdpE, encoding the transcriptional response regulator, as well as of the downstream gene, orfX, in adaptation of L. monocytogenes EGD to NaCl and low temperature. When grown in chemically defined medium the addition of NaCl to 2% decreased the growth rate of a mutant with an insertional inactivated kdpE, while mutants carrying in-frame deletions of either kdpE or orfX were unaffected by high osmolarity. Transcriptional analysis of kdpE and orfX revealed that their products are encoded by the same transcript. Thus, our data indicate that the absence of both KdpE and OrfX influences growth under osmotic pressure. Interestingly, OrfX contains a conserved domain of alpha/beta-hydrolases and resembles RsbQ that in Bacillus subtilis participates in the activation cascade of the general stress sigma factor SigB. When shifted to low temperature the deletion mutant lacking orfX resumed growth slightly faster than the wild-type. This phenotype was shared by a mutant carrying an in-frame deletion of sigB supporting the notion that OrfX could be a RsbQ homologue.