AUTHOR=dos Santos Patrícia T. , Menendez-Gil Pilar , Sabharwal Dharmesh , Christensen Jens-Henrik , Brunhede Maja Z. , Lillebæk Eva M. S. , Kallipolitis Birgitte H. 

TITLE=The Small Regulatory RNAs LhrC1–5 Contribute to the Response of Listeria monocytogenes to Heme Toxicity

JOURNAL=Frontiers in Microbiology

VOLUME=9

YEAR=2018

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2018.00599

DOI=10.3389/fmicb.2018.00599

ISSN=1664-302X

ABSTRACT=<p>The LhrC family of small regulatory RNAs (sRNAs) is known to be induced when the foodborne pathogen <italic>Listeria monocytogenes</italic> 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 <italic>lhrC1–5</italic> 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 <italic>L. monocytogenes</italic> 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: <italic>oppA</italic>, <italic>tcsA,</italic> and <italic>lapB</italic>, 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 <italic>L. monocytogenes</italic>. We also investigated the role of LhrC1–5 in controlling the expression of genes involved in heme uptake and utilization: <italic>lmo2186</italic> and <italic>lmo2185</italic>, encoding the heme-binding proteins Hbp1 and Hbp2, respectively, and <italic>lmo0484</italic>, encoding a heme oxygenase-like protein. Using <italic>in vitro</italic> binding assays, we demonstrated that the LhrC family member LhrC4 interacts with mRNAs encoded from <italic>lmo2186</italic>, <italic>lmo2185,</italic> and <italic>lmo0484</italic>. For <italic>lmo0484</italic>, 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 <italic>lmo0484</italic> 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 <italic>L. monocytogenes</italic>.</p>