How Quorum Sensing Connects Sporulation to Necrotrophism in Bacillus thuringiensis

Abstract

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.

Structural Insights into Streptococcal Competence Regulation by the Cell-to-Cell Communication System ComRS

“…Finally, binding of the NprX pheromone to NprR has been shown to stabilize a tetrameric conformation without similarity with the subunit interaction modes of the other RNPP transcriptional regulators [20]. Interestingly, NprR is a bifunctional quorum sensor, acting as a Rap-like phosphatase in the absence of peptide [21]. The apo form of NprR inhibits sporulation by binding to the phosphotransferase Spo0F, in the same way as observed with RapH [16].…”

“…Except for the Rap phosphatases, RNPP proteins are transcription factors displaying an N-terminal HTH domain, which binds to specific DNA motifs [11, 12]. Structure-function studies have been undertaken for all members of the family: PrgX [13, 14], PlcR [5, 15], Rap [16–19], NprR [20, 21]. In all cases, pheromone binding to the TPR domain induces conformational changes that regulate their activity.…”

Bacillus thuringiensis targets the host intestinal epithelial junctions for successful infection of Caenorhabditis elegans

“…Analysis of the nematicidal activity of these Bt strains against the wild type N2 and the mutant clc‐1(ok2500) worms showed that both nematodes were more resistance to the infection with Δ plcR and Δ nprR Bt strains compared with the BMB0215 parental strain ( t‐ test, p < 0.05) (Fig. B), supporting that PlcR and NprR regulons are controlling the expression of virulence factors that contribute to the Bt pathogenicity (Perchat et al ., ). We also observed that BMB0215 and mutant Δ nprR Bt strains showed significant higher toxicity against the mutant clc‐1(ok2500) than to the wild type N2 worms ( t‐ test, p < 0.05).…”

“…This protein regulates a set of extracellular toxins and enzymes (Agaisse et al ., ; Salamitou et al ., ; Gohar et al ., ). The quorum‐sensing regulator NprR mainly controls the necrotrophic process of the bacteria in insects, by regulating expression of degradative enzymes (Slamti et al ., ; Perchat et al ., ). Although in nematodes the role of PlcR and NprR in Bt infection still remains to be analysed, we showed here that the PlcR is required for Bt disruption of the epithelial junction in C. elegans , while the NprR regulator is not involved (Fig.…”

“…As a pathogen, Bt could complete a full infection life cycle in both insects and nematodes hosts (Raymond et al, 2010;Ruan et al, 2015). Overall, the infection process of Bt in insects includes three successive steps: virulence, necrotrophic and sporulation processes (Perchat et al, 2016). The early events in pathogenesis corresponds to the virulence process, just after ingestion by susceptible hosts, Cry toxins form pores in the host intestinal cell membranes, inducing favourable conditions for spore germination.…”