Date Published: February 21, 2013
Publisher: Public Library of Science
Author(s): Arnaud Firon, Asmaa Tazi, Violette Da Cunha, Sophie Brinster, Elisabeth Sauvage, Shaynoor Dramsi, Douglas T. Golenbock, Philippe Glaser, Claire Poyart, Patrick Trieu-Cuot, Robin Charles May.
Group B Streptococcus (GBS), a common commensal of the female genital tract, is the leading cause of invasive infections in neonates. Expression of major GBS virulence factors, such as the hemolysin operon cyl, is regulated directly at the transcriptional level by the CovSR two-component system. Using a random genetic approach, we identified a multi-spanning transmembrane protein, Abx1, essential for the production of the GBS hemolysin. Despite its similarity to eukaryotic CaaX proteases, the Abx1 function is not involved in a post-translational modification of the GBS hemolysin. Instead, we demonstrate that Abx1 regulates transcription of several virulence genes, including those comprising the hemolysin operon, by a CovSR-dependent mechanism. By combining genetic analyses, transcriptome profiling, and site-directed mutagenesis, we showed that Abx1 is a regulator of the histidine kinase CovS. Overexpression of Abx1 is sufficient to activate virulence gene expression through CovS, overcoming the need for an additional signal. Conversely, the absence of Abx1 has the opposite effect on virulence gene expression consistent with CovS locked in a kinase-competent state. Using a bacterial two-hybrid system, direct interaction between Abx1 and CovS was mapped specifically to CovS domains involved in signal processing. We demonstrate that the CovSR two-component system is the core of a signaling pathway integrating the regulation of CovS by Abx1 in addition to the regulation of CovR by the serine/threonine kinase Stk1. In conclusion, our study reports a regulatory function for Abx1, a member of a large protein family with a characteristic Abi-domain, which forms a signaling complex with the histidine kinase CovS in GBS.
Some commensal microorganisms are also opportunistic pathogens. Harmless, and potentially beneficial, they may become causative agents of local or systemic infections , . To date, the signals dictating the switch from commensalism to virulence are mainly unknown. The complex set of genetic and environmental factors thought to be involved would affect the equilibrium between the host and the microbes. Deciphering the molecular events that govern the transition between commensalism and virulence will contribute to understanding and controlling infections due to opportunistic pathogens.
The CovSR two-component system (TCS) is the major regulator of virulence gene expression in GBS. In this study, we identified the Abx1 transmembrane protein as essential and limiting for CovSR activity due to its interaction with the CovS histidine kinase (HK). Regulation of HK activity by an interacting protein is increasingly recognized when studying TCS signaling in a cellular context –. Instead of being a linear signaling pathway, our data show that the CovSR TCS is embedded in a regulatory network involving several regulatory proteins, including the Abi-domain protein Abx1.