Downregulation of motility during stress requires stressosome input in Listeria monocytogenes strain EGD-e

Motility genes in Listeria monocytogenes are expressed under saprophytic conditions (30°C or less) but are repressed within the host at 37°C. Motility is costly on cellular resources due to the large molecular structures that need to be synthesized, and the proton motive force required for flagellar rotation. 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 is unable to inactivate the stressosome, the sensory hub at the top of the SigB activation pathway, results in motility repression. Escape from this repressed state occurred through the acquisition of spontaneous mutations that decreased SigB activity. Flagellar expression was abolished in strains lacking RsbX, and a transcriptomic analysis revealed that the entire flagellar operon was strongly repressed, including the motility anti-repressor gene gmaR. These effects could be reversed by providing functional copies of rsbX or gmaR in trans. Abolishing expression of an antisense RNA lying opposite the large flagella operon or deleting the mogR transcriptional repressor restored the ability to produce flagellin in the ΔrsbX background. Stressosome mutations that negatively affect SigB activity resulted in a derepressed motility phenotype, whereas those that increase SigB activity resulted in a decreased motility phenotype similar to the ΔrsbX strain. Our data indicate that stress sensing via the stressosome negatively impacts motility and shows that the general stress response is prioritized when L. monocytogenes encounters osmotic and light stress conditions.IMPORTANCEMotility involves the synthesis and operation of the flagella, which come at a high energy cost for the bacterium and need to be carefully controlled. The human pathogen Listeria monocytogenes senses and responds to various stresses, in part through the alternative sigma factor σ^B (SigB), which controls the general stress response regulon. Since the SigB-regulon harbor hundreds of genes, the activity of SigB needs to be carefully controlled under non-stressed conditions to save energy. On the other hand, upon stress, the bacterium needs to invest a large amount of energy to synthesize a myriad of proteins to cope with the increased stress. In this study, we examined how motility is regulated under osmotic and visible light stress. Our data imply that increased SigB activity negatively impacts motility gene expression by a signal that is conveyed through the stressosome multiprotein complex.