The Small Regulatory RNAs LhrC1-5 Contribute to the Response of to Heme Toxicity.

The LhrC family of small regulatory RNAs (sRNAs) is known to be induced when the foodborne pathogen is exposed to infection-relevant conditions, such as human blood. Here we demonstrate that excess heme, the core component of hemoglobin in blood, leads to a strong induction of the LhrC family members LhrC1-5. The heme-dependent activation of relies on the response regulator LisR, which is known to play a role in virulence and stress tolerance. Importantly, our studies revealed that LhrC1-5 and LisR contribute to the adaptation of to excess heme. Regarding the regulatory function of the sRNAs, we demonstrate that LhrC1-5 act to down-regulate the expression of known LhrC target genes under heme-rich conditions: , and , encoding surface exposed proteins with virulence functions. These genes were originally identified as targets for LhrC-mediated control under cell envelope stress conditions, suggesting a link between the response to heme toxicity and cell envelope stress in . We also investigated the role of LhrC1-5 in controlling the expression of genes involved in heme uptake and utilization: and , encoding the heme-binding proteins Hbp1 and Hbp2, respectively, and , encoding a heme oxygenase-like protein. Using binding assays, we demonstrated that the LhrC family member LhrC4 interacts with mRNAs encoded from , and . For , we furthermore show that LhrC4 uses a CU-rich loop for basepairing to the AG-rich Shine-Dalgarno region of the mRNA. The presence of a link between the response to heme toxicity and cell envelope stress was further underlined by the observation that LhrC1-5 down-regulate the expression of in response to the cell wall-acting antibiotic cefuroxime. Collectively, this study suggests a role for the LisR-regulated sRNAs LhrC1-5 in a coordinated response to excess heme and cell envelope stress in .