A Small RNA Controls Expression of the Chitinase ChiA in <em>Listeria monocytogenes</em>

<div><p>In recent years, more than 60 small RNAs (sRNAs) have been identified in the gram-positive human pathogen <em>Listeria monocytogenes</em>, but their putative roles and mechanisms of action remain largely unknown. The sRNA LhrA was recently shown to be a post-transcriptional regulator of a single gene, <em>lmo0850</em>, which encodes a small protein of unknown function. LhrA controls the translation and degradation of the <em>lmo0850</em> mRNA by an antisense mechanism, and it depends on the RNA chaperone Hfq for efficient binding to its target. In the present study, we sought to gain more insight into the functional role of LhrA in <em>L. monocytogenes</em>. To this end, we determined the effects of LhrA on global-wide gene expression. We observed that nearly 300 genes in <em>L. monocytogenes</em> are either positively or negatively affected by LhrA. Among these genes, we identified <em>lmo0302</em> and <em>chiA</em> as direct targets of LhrA, thus establishing LhrA as a multiple target regulator. <em>Lmo0302</em> encodes a hypothetical protein with no known function, whereas <em>chiA</em> encodes one of two chitinases present in <em>L. monocytogenes.</em> We show here that LhrA acts as a post-transcriptional regulator of <em>lmo0302</em> and <em>chiA</em> by interfering with ribosome recruitment, and we provide evidence that both LhrA and Hfq act to down-regulate the expression of <em>lmo0302</em> and <em>chiA</em>. Furthermore, <em>in vitro</em> binding experiments show that Hfq stimulates the base pairing of LhrA to <em>chiA</em> mRNA. Finally, we demonstrate that LhrA has a negative effect on the chitinolytic activity of <em>L. monocytogenes</em>. In marked contrast to this, we found that Hfq has a stimulating effect on the chitinolytic activity, suggesting that Hfq plays multiple roles in the complex regulatory pathways controlling the chitinases of <em>L. monocytogenes</em>.</p> </div>