Downregulation of motility during stress conditions requires stressosome input in Listeria monocytogenes
Motility in Listeria monocytogenes is negatively regulated within the mammalian host in response to temperature through a complex interplay between positively acting and negatively acting regulators. Motility genes are expressed under saprophytic conditions where the temperature is 30ºC or less. Motility is costly on cellular resources due to the large molecular structures that need to be synthesized, and the proton motive force required to produce flagellar rotation. Under stressful environmental conditions there is a trade off between motility and energy conservation and bacteria typically repress motility when adverse conditions are encountered. Here we investigated the impact of the SigB-mediated general stress response on the regulation of motility in L. monocytogenes and sought to elucidate the regulatory steps involved. We show that an rsbX mutation that compromises the ability to inactivate the stressosome, the sensory hub at the top of the SigB activation pathway, results in motility repression. Escaping from this repressed state occurred through the acquisition of mutations that decreased SigB activity. Flagellar expression was abolished in these mutants and a transcriptomic analysis revealed that the entire flagellar operon is strongly repressed. The motility anti-repressor gene gmaR was also downregulated in an ?rsbX background. These effects can be reversed by providing functional copies of rsbX or gmaR in trans. Deletion of an internal SigB dependent promoter that produces an antisense RNA for the large motility operon (including gmaR) restores the ability to produce flagellin in the ?rsbX background. A similar phenotype is observed by deleting MogR, which acts as a negative regulator of flagella motility. Stressosome mutations that negatively affect SigB activity   result in derepressed motility whereas those that increase SigB activity result in a decreased motility phenotype similar to the ?rsbX strain. Overall, the data indicate that stress sensing via the stressosome negatively impacts motility in L. monocytogenes and shows that the general stress response can take priority over motility when L. monocytogenes encounters harsh environmental conditions. Overall design: RNA-seq profiles of the wild-type and ?rsbX strains of L. monocytogenes EGD-e.  Bacteria  were grown in BHI medium at 23°C under normal laboratory light conditions until OD600 = 0.85 when bacteria were harvested, RNA was isolated and subjected to RNA-seq.