Date Published: August 2, 2016
Publisher: Public Library of Science
Author(s): Stéphane Perchat, Antoine Talagas, Sandrine Poncet, Noureddine Lazar, Inès Li de la Sierra-Gallay, Michel Gohar, Didier Lereclus, Sylvie Nessler, Theresa M. Koehler.
Bacteria use quorum sensing to coordinate adaptation properties, cell fate or commitment to sporulation. The infectious cycle of Bacillus thuringiensis in the insect host is a powerful model to investigate the role of quorum sensing in natural conditions. It is tuned by communication systems regulators belonging to the RNPP family and directly regulated by re-internalized signaling peptides. One such RNPP regulator, NprR, acts in the presence of its cognate signaling peptide NprX as a transcription factor, regulating a set of genes involved in the survival of these bacteria in the insect cadaver. Here, we demonstrate that, in the absence of NprX and independently of its transcriptional activator function, NprR negatively controls sporulation. NprR inhibits expression of Spo0A-regulated genes by preventing the KinA-dependent phosphorylation of the phosphotransferase Spo0F, thus delaying initiation of the sporulation process. This NprR function displays striking similarities with the Rap proteins, which also belong to the RNPP family, but are devoid of DNA-binding domain and indirectly control gene expression via protein-protein interactions in Bacilli. Conservation of the Rap residues directly interacting with Spo0F further suggests a common inhibition of the sporulation phosphorelay. The crystal structure of apo NprR confirms that NprR displays a highly flexible Rap-like structure. We propose a molecular regulatory mechanism in which key residues of the bifunctional regulator NprR are directly and alternatively involved in its two functions. NprX binding switches NprR from a dimeric inhibitor of sporulation to a tetrameric transcriptional activator involved in the necrotrophic lifestyle of B. thuringiensis. NprR thus tightly coordinates sporulation and necrotrophism, ensuring survival and dissemination of the bacteria during host infection.
Sporulating bacteria have developed a number of sophisticated mechanisms devoted to the temporal and spatial coordination of cell fate and gene expression. Regulatory mechanisms, as two-component systems, contribute to maintain this tight coordination by tuning gene expression and protein activation in response to a large variety of environmental stimuli including nutrient limitation and population density . In Gram-positive bacteria, quorum sensing is a mode of cell-cell communication involving the secretion of diffusible signaling peptides recognized by the responder bacteria [2, 3]. These peptides elicit a response either directly, by binding to effector proteins or indirectly, by triggering a two-component phosphorelay.
Unlike the results published by Cabrera and colleagues  claiming that NprR has a positive effect on sporulation, our results demonstrate that NprR is a bifunctional regulator repressing sporulation in the absence of its cognate peptide NprX and acting as a transcriptional activator in the presence of peptide. These findings are in agreement with previous studies suggesting that NprR is an evolutionary intermediate between the Rap and the transcriptional activators of the RNPP family [4, 8].