A molecular genetic investigation into stress sensing in the food-borne pathogen Listeria Monocytogenes: roles for RsbR and its paralogues
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The alternative sigma factor σB is conserved across several Gram-positive bacteria species as the major general stress response (GSR) regulator. For the food-borne pathogen Listeria monocytogenes, σB enables the bacterium to persist in growth-limiting environments, thus posing a serious concern for food processing industries. Activation of σB is dependent on signalling from a multiprotein stress-sensing complex known as the stressosome. Here, we investigate the individual roles of the five identified stress sensor proteins: RsbR, the blue-light photoreceptor Lmo0799, Lmo0161, Lmo1642 and Lmo1842 which are thought to form a multi-protein stressosome complex. Mutant strains were constructed or obtained, each lacking RsbR or one of its-paralogues and were subjected to a number of phenotypic tests. Blue (460-470 nm) light was found to have a clear inhibitory effect on growth. Removal of the Lmo0799 protein, whose light-sensing function was proven to be dependent on a conserved cysteine residue at position 56, did not affect σB-mediated survival in the presence of higher intensity blue light. Phenotypic test results suggested that there may be redundancy in stress sensing between several of the RsbR paralogues but the RsbR protein plays a core structural role in stressosome formation. Novel phenotypes initially observed for the Δlmo1842 and Δlmo1642 strains were found to be caused by the presence of an RsbV I23T amino acid replacement which prevented σB activation at 37 °C but not 30 °C. Additionally, we identified that at low levels of stress, the ΔsigB mutant displayed a growth advantage over the wild-type, highlighting the energy cost associated with activating the GSR. These findings provide new insight into the mechanisms by which L. monocytogenes senses and responds to its environment and may have potential implications for control of this pathogen in food environments.