The human pathogens Streptococcus pneumoniae and Streptococcus mutans have both evolved complex quorum sensing (QS) systems that regulate the production of bacteriocins and the entry into the competent state, a requirement for natural transformation. Natural transformation provides bacteria with a mechanism to repair damaged genes or as a source of new advantageous traits. In S. pneumoniae, the competence pathway is controlled by the two-component signal transduction pathway ComCDE, which directly regulates SigX, the alternative sigma factor required for the initiation into competence. Over the past two decades, effectors of cellular killing (i.e., fratricides) have been recognized as important targets of the pneumococcal competence QS pathway. Recently, direct interactions between the ComCDE and the paralogous BlpRH pathway, regulating bacteriocin production, were identified, further strengthening the interconnections between these two QS systems. Interestingly, a similar theme is being revealed in S. mutans, the primary etiological agent of dental caries. This review compares the relationship between the bacteriocin and the competence QS pathways in both S. pneumoniae and S. mutans, and hopes to provide clues to regulatory pathways across the genus Streptococcus as a potential tool to efficiently investigate putative competence pathways in nontransformable streptococci.

Natural transformation, or competence, is an ability inherent to bacteria for the uptake of extracellular DNA. This process is central to bacterial evolution and allows for the rapid acquirement of new traits, such as antibiotic resistance in pathogenic microorganisms. For the Gram-positive bacteria genus Streptococcus, genes required for competence are under the regulation of quorum sensing (QS) mediated by peptide pheromones. One such system, ComRS, consists of a peptide (ComS) that is processed (XIP), secreted, and later imported into the cytoplasm, where it binds and activates the transcription factor ComR. ComR then engages in a positive feedback loop for the expression of ComS and the alternative sigma-factor SigX. Although ComRS are present in the majority of Streptococcus species, the sequence of both ComS/XIP and ComR diverge significantly, suggesting a mechanism for species-specific communication. To study possible cross-talk between streptococcal species in the regulation of competence, and to explore in detail the molecular interaction between ComR and XIP we undertook an interdisciplinary approach. We developed a ‘test-bed’ assay to measure the activity of different ComR proteins in response to cognate and heterologous XIP peptides in vivo, revealing distinct ComR classes of strict, intermediate, and promiscuous specificity among species. We then solved an X-ray crystal structure of ComR from S. suis to further understand the interaction with XIP and to search for structural features in ComR proteins that may explain XIP recognition. Using the structure as a guide, we probed the apo conformation of the XIP-binding pocket by site-directed mutagenesis, both in test-bed cultures and biochemically in vitro. In alignments with ComR proteins from other species, we find that the pocket is lined by a variable and a conserved face, where residues of the conserved face contribute to ligand binding and the variable face discriminate among XIP peptides. Together, our results not only provide a model for XIP recognition and specificity, but also allow for the prediction of novel XIP peptides that induce ComR activity.

No abstract