The HtrA Protease of Streptococcus pneumoniae Controls Density-Dependent Stimulation of the Bacteriocin blp Locus via Disruption of Pheromone Secretion

Abstract

All fully sequenced strains of Streptococcus pneumoniae (pneumococcus) contain a version of the blp locus which is responsible for the regulation and secretion of a variable repertoire of pneumococcal bacteriocins called pneumocins and their associated immunity proteins. Pneumocins mediate intra-and interspecies competition in vitro and have been shown to provide a competitive advantage in vivo. Pneumocin production is stimulated by extracellular accumulation of the peptide pheromone, BlpC. Both BlpC and the functional pneumocins are secreted out of the cell via the Blp transporter, BlpAB. The conserved surface-expressed serine protease, HtrA, has been shown to limit activation of the locus and secretion of functional pneumocins. In this work, we demonstrate that htrA mutants stimulate the blp locus at lower cell density and to a greater extent than strains expressing wild-type HtrA. This effect is not due to direct proteolytic degradation of secreted pheromone by the protease, but instead is a result of HtrA-mediated disruption of peptide processing and secretion. Because pneumocins are secreted through the same transporter as the pheromone, this finding explains why pheromone supplementation cannot completely restore pneumocin inhibition to strains expressing high levels of HtrA despite restoration of blp transcriptional activity. HtrA restricts pneumocin production to high cell density by limiting the rate of accumulation of BlpC in the environment. Importantly, HtrA does not interfere with the ability of a strain to sense environmental pheromones, which is necessary for the induction of protective immunity in the face of pneumocin-secreting competitors.

Mutation of the Thiol-Disulfide Oxidoreductase SdbA Activates the CiaRH Two-Component System, Leading to Bacteriocin Expression Shutdown in Streptococcus gordonii

“…For example, CiaH positively regulates bacteriocin production in S. mutans, and ⌬ciaH mutants lack mutacin activity (22). In contrast, similar to our findings in S. gordonii, the CiaRH system in S. pneumoniae represses bacteriocin activity (17,18). In S. pneumoniae, the CiaRH system inhibits bacteriocin production through HtrA (DegP), which has been shown to disrupt the BlpAB transporter required for bacteriocin processing and secretion.…”

“…In S. pneumoniae, the CiaRH system inhibits bacteriocin production through HtrA (DegP), which has been shown to disrupt the BlpAB transporter required for bacteriocin processing and secretion. Thus, inactivation of HtrA can alleviate CiaRH-mediated repression and increase the amounts of secreted bacteriocins (18).…”

“…Streptococcus pneumoniae produces two bacteriocins encoded by the blp locus, which are controlled by a dedicated quorum-sensing and secretion system; however, the activity of the Blp system is also modulated by at least two additional regulatory systems, namely, ComDE (16) and CiaRH (17). Although the mechanisms involved are not fully understood, the serine protease HtrA (DegP) also appears to play an important role in regulating S. pneumoniae bacteriocin production (17,18). Similarly, S. mutans produces at least 10 different bacteriocins, which vary by strain, include both lantibiotics and nonlantibiotics (19), and are subject to regulation by complex overlapping systems, including ComDE (20,21), CiaRH (22), VicRK (23), HdrRM (24), and BrsRM (25).…”

An Electrostatic Interaction between BlpC and BlpH Dictates Pheromone Specificity in the Control of Bacteriocin Production and Immunity in Streptococcus pneumoniae

“…To control the potential for cross stimulation, at least four different alleles of the blpC gene encode the peptide pheromone, and these correspond to the four major alleles of the blpH gene encoding the pheromone receptor (13,14,16,18). The BlpC peptides are characterized by a highly conserved N-terminal signal sequence region followed by a double glycine motif.…”

“…The signal sequence is cleaved from the active peptide during transport by the peptidase domain of the BlpA transporter. Genomes that encode a particular blpC allele typically encode the same histidine kinase allele, suggesting that a particular histidine kinase type will respond only to its cognate pheromone type (13,14,18). The BlpH receptor is a member of the class 10 receptor-histidine protein kinase family that includes ComD, involved in the induction of the pneumococcal competence state, and AgrC, involved in the regulation of virulence factors in Staphylococcus aureus.…”