Quorum sensing in group A Streptococcus

Abstract

Quorum sensing (QS) is a widespread phenomenon in the microbial world that has important implications in the coordination of population-wide responses in several bacterial pathogens. In Group A Streptococcus (GAS), many questions surrounding QS systems remain to be solved pertaining to their function and their contribution to the GAS lifestyle in the host. The QS systems of GAS described to date can be categorized into four groups: regulator gene of glucosyltransferase (Rgg), Sil, lantibiotic systems, and LuxS/AI-2. The Rgg family of proteins, a conserved group of transcription factors that modify their activity in response to signaling peptides, has been shown to regulate genes involved in virulence, biofilm formation and competence. The sil locus, whose expression is regulated by the activity of signaling peptides and a putative two-component system (TCS), has been implicated on regulating genes involved with invasive disease in GAS isolates. Lantibiotic regulatory systems are involved in the production of bacteriocins and their autoregulation, and some of these genes have been shown to target both bacterial organisms as well as processes of survival inside the infected host. Finally AI-2 (dihydroxy pentanedione, DPD), synthesized by the LuxS enzyme in several bacteria including GAS, has been proposed to be a universal bacterial communication molecule. In this review we discuss the mechanisms of these four systems, the putative functions of their targets, and pose critical questions for future studies.

Keywords: AI-2; Rgg; Sil; Streptococcus pyogenes; cell-cell signaling; lantibiotics; pheromones; quorum sensing.

Figure 1

Quorum sensing signaling in Gram positive bacteria. After being translated by the ribosome, pre-peptides are processed, and exported from the cell to generate an active signaling pheromone. Pheromones accumulate in the extracellular medium where they can be detected by the producer cell and neighboring bacteria. Pheromone detection can either occur through two-component systems in the bacterial membrane (left side) or by direct binding by transcription factors after peptide import (right side). After pheromone detection, the activated response regulator or the pheromone-bound transcription factor induce changes in target gene expression, and genes encoding for pheromone pre-peptides are up-regulated, increasing pheromone production and generating an autoinduction process.

Figure 2

Rgg regulators of Streptococcus pyogenes. (A) RopB directly activates the expression of speB and its associated downstream genes, while directly repressing the prophage encoded spd3 DNase. RopB also affects, by direct and indirect manners, the expression of a varying group of genes in different isolates. Other factors can modulate RopB activity, like LacD.1, the N-terminal peptide of the Vfr protein and unknown factors imported by the Opp and Dpp permeases. (B) The Rgg2/3 system. Left panel: in the absence of SHPs, Rgg3 is bound to the promoter region of the pheromone genes, inhibiting their expression. Addition of exogenous SHP pheromones that bind Rgg3, cause its release from DNA, while allowing Rgg2 to bind the same promoter region and promote expression of pheromone genes. Right panel: activation of the Rgg2/3 system triggers expression of shp2, shp3 and their downstream genes. Translated SHP2 and SHP3 pro-peptides are secreted through an unknown exporter and processed by the activity of Eep and additional extracellular enzymes. The active SHP2-C8 and SHP3-C8 pheromones are imported via Opp to complete the autoinduction loop. (C) Regulation of competence genes by ComR. ComS pro-peptide is produced, secreted and processed to generate the XIP peptide. After being imported through Opp, XIP can bind ComR, which binds to the promoters of comS and sigX to activate their expression. The alternative sigma factor σX together with RNA polymerase bind to Com box sites in target genes and activate the expression of competence related genes. Promoters with open arrows are Rgg-independent. Promoters with filled arrows are activated by Rgg-proteins. Promoters with X symbol are repressed by Rgg-proteins.

Figure 3

Sil signaling system. (A) Model of signaling. SilCR pre-peptide is produced, secreted and processed. Mature SilCR is detected by SilB, activating the response regulator SilA which activates expression from select promoters, including the promoter for the expression of silED-CR genes. Expression of the sil locus is also in a unknown manner by a second two-component system, TrxSR. Asparagine sensing by TrxS alleviates repression of target genes by TrxR, generating an increase in expression of SilCR dependent promoters. (B) sil genomic island. The sil locus plus neighboring genes are located in a putative genomic island in the strain JS95. Chromosomal location is compared with the SF370 strain that does not possess the sil locus.

Figure 4

Lantibiotic regulatory systems. (A) Model of production and sensing. Lantibiotic pre-peptide is synthesized, exported, cleaved, and modified through thiosulfur bridge formation and aminoacid dehydration. The mature lantibiotic can exert its antibacterial effect on sensitive bacteria either by targeting the cytoplasmic membrane or inhibiting activity of target enzymes. Lantibiotic-producing bacteria express membrane immunity proteins to bind their cognate lantibiotic, or to re-export via an ABC transporter. Two-component systems sense the lantibiotic and trigger activation of the lantibiotic gene cluster in producing and neighboring bacteria. (B) Lantibiotic gene clusters present in Streptococcus pyogenes. Complete or partial components of lantibiotic genes in sequenced strains of GAS compared with reference strains (in bold) for sal cluster (S. salivarius 20P3, accession AY005472), and srt cluster (S. pyogenes BL-T, accession AB030831).

Figure 5

AI-2 production and sensing. (A) Reactive methyl cycle and production of DPD. SAM-dependent methyltransferases convert SAM to SAH, accumulation of which confers product-feedback inhibition on methyltransferase reactions. SAH is detoxified to SRH by Pfs. SRH is converted to homocysteine and DPD by LuxS. Homocysteine can be recycled to SAM via MetH, which generates methionine, and MetK. DPD spontaneously cyclizes and undergoes ketone hydration to form R- and S-THMF. The latter is also found in its borated-form, S-THMF-borate (reproduced and modified with permission from Federle, 2009) (B) The two families of known AI-2 receptors, LuxPQ, and LsrB, which sense extracelullar AI-2 or import AI-2 to bind LsrR, respectively. LuxPQ binds the S-THMF-borate form of AI-2, while LsrB binds the R-THMF form.