Date Published: March 20, 2014
Publisher: Public Library of Science
Author(s): Cédric Romilly, Claire Lays, Arnaud Tomasini, Isabelle Caldelari, Yvonne Benito, Philippe Hammann, Thomas Geissmann, Sandrine Boisset, Pascale Romby, François Vandenesch, Michael S. Gilmore.
Staphylococcus aureus produces a high number of RNAs for which the functions are poorly understood. Several non-coding RNAs carry a C-rich sequence suggesting that they regulate mRNAs at the post-transcriptional level. We demonstrate that the Sigma B-dependent RsaA RNA represses the synthesis of the global transcriptional regulator MgrA by forming an imperfect duplex with the Shine and Dalgarno sequence and a loop-loop interaction within the coding region of the target mRNA. These two recognition sites are required for translation repression. Consequently, RsaA causes enhanced production of biofilm and a decreased synthesis of capsule formation in several strain backgrounds. These phenotypes led to a decreased protection of S. aureus against opsonophagocytic killing by polymorphonuclear leukocytes compared to the mutant strains lacking RsaA. Mice animal models showed that RsaA attenuates the severity of acute systemic infections and enhances chronic catheter infection. RsaA takes part in a regulatory network that contributes to the complex interactions of S. aureus with the host immune system to moderate invasiveness and favour chronic infections. It is the first example of a conserved small RNA in S. aureus functioning as a virulence suppressor of acute infections. Because S. aureus is essentially a human commensal, we propose that RsaA has been positively selected through evolution to support commensalism and saprophytic interactions with the host.
Staphylococcus aureus is an opportunistic pathogen that has evolved complex regulatory circuits allowing rapid adaption of cell growth in response to its diverse hosts and ecological niches. Present in a large proportion of the population as a commensal of skin and nose, the bacteria is also responsible for a large range of hospital-acquired and community infections . A successful infection by S. aureus largely depends on the coordinated and sequential expression of a multitude of virulence factors and accessory genes. Over the last decade, it has been established that S. aureus genes are regulated at many different levels by a variety of trans-acting regulators, which act in a coordinated manner , . Among them, RNAs are now recognized as important players in virulence and many physiological and adaptive responses , . The first regulatory RNA that was discovered in 1993 is RNAIII, the main intracellular effector of the quorum sensing agr system . This multi-functional regulatory RNA binds to several target mRNAs to regulate their translation and decay –. Later on, several teams have experimentally identified a large number of small RNAs (sRNA) that are issued from the core genome and from mobile and accessory elements (e.g., , ). These sRNAs include cis-acting regulatory regions of mRNAs (the so-called riboswitches), cis-encoding antisense RNAs (asRNA), and non-coding RNAs (ncRNAs). In addition, sRNAs carrying small open reading frames (sORF) have been recently identified – and one of them was shown to express a small cytolytic peptide . The functional and mechanistic studies of trans-acting sRNAs are still lagging behind their discovery, but some of them are known to sense population density and various environmental changes, modify cell surface properties, adjust the bacterial metabolism during cell growth, regulate virulence gene expression, and respond to antibiotic treatment , .