SummaryAll streptococcal genomes encode the alternative sigma factor SigX and 21 SigX-dependent proteins required for genetic transformation, yet no pyogenic streptococci are known to develop competence. Resolving this paradox may depend on understanding the regulation of sigX. We report the identification of a regulatory circuit linked to the sigX genes of mutans, pyogenic, and bovis streptococci that uses a novel small, double-tryptophan-containing sigX-inducing peptide (XIP) pheromone. In all three groups, the XIP gene (comS), and sigX have identical, non-canonical promoters consisting of 9 bp inverted repeats separated from a -10 hexamer by 19 bp. comS is adjacent to a gene encoding a putative transcription factor of the Rgg family and is regulated by its product, which we designate ComR. Deletion of comR or comS in Streptococcus mutans abolished transformability, as did deletion of the oligopeptide permease subunit oppD, suggesting that XIP is imported. Providing S. mutans with synthetic fragments of ComS revealed that seven C-terminal residues, including the WW motif, cause robust induction of both sigX and the competent state. We propose that this circuit is the proximal regulator of sigX in S. mutans, and we infer that it controls competence in a parallel way in all pyogenic and bovis streptococci.

SummaryAlthough the observation that Gram-negative bacteria produce outer membrane vesicles (MVs) was made over 40 years ago, their biological roles have become a focus of study only within the past 10 years. Recent progress in this area has revealed that bacterial MVs are utilized for several processes including delivery of toxins to eukaryotic cells, protein and DNA transfer between bacterial cells, and trafficking of cell-cell signals. Some of these roles appear to be generalized among the Gram-negative bacteria while others are restricted to specific bacterial species/strains. Here we review the known roles of MVs, propose other roles for MVs in mediating interspecies and interkingdom communication, and discuss the mechanism of MV formation.

SummaryBacteria have evolved elaborate communication strategies to co-ordinate their group activities, a process termed quorum sensing (QS). Pseudomonas aeruginosa is an opportunistic pathogen that utilizes QS for diverse activities, including disease pathogenesis. P. aeruginosa has evolved a novel communication system in which the signal molecule 2-heptyl-3-hydroxy-4-quinolone (Pseudomonas Quinolone Signal, PQS) is trafficked between cells via membrane vesicles (MVs). Not only is PQS packaged into MVs, it is required for MV formation. Although MVs are involved in important biological processes aside from signalling, the molecular mechanism of MV formation is unknown. To provide insight into the molecular mechanism of MV formation, we examined the interaction of PQS with bacterial lipids. Here, we show that PQS interacts strongly with the acyl chains and 4Ј-phosphate of bacterial lipopolysaccharide (LPS). Using PQS derivatives, we demonstrate that the alkyl side-chain and third position hydroxyl of PQS are critical for these interactions. Finally, we show that PQS stimulated purified LPS to form liposome-like structures. These studies provide molecular insight into P. aeruginosa MV formation and demonstrate that quorum signals serve important non-signalling functions.

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