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Chapter 2 : Extracellular Peptide Signaling and Quorum Responses in Development, Self-Recognition, and Horizontal Gene Transfer in Bacillus subtilis
Category: Microbial Genetics and Molecular Biology; Bacterial Pathogenesis
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Extracellular Peptide Signaling and Quorum Responses in Development, Self-Recognition, and Horizontal Gene Transfer in Bacillus subtilis, Page 1 of 2< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555815578/9781555814045_Chap02-1.gif /docserver/preview/fulltext/10.1128/9781555815578/9781555814045_Chap02-2.gif
This chapter focuses on the mechanisms of extracellular peptide signaling that Bacillus subtilis utilizes to control gene expression involved in sporulation, the ComA-mediated general quorum response, and horizontal gene transfer. This signaling provides mechanisms for the cell to monitor population density and to distinguish whether neighboring cells are similar or different. All the responses described are stimulated by high population densities. The focus of the chapter is cell-cell signaling mediated by peptides. Horizontal gene transfer plays an important role in bacterial evolution. It is often regulated by cell-cell signaling, including the pheromone-responsive conjugal plasmids of Entercoccus faecalis, the autoinducer-sensing conjugal plasmids of Agrobacterium tumefaciens, and competence development in Streptococcus and Bacillus species. Self-recognition during B. subtilis competence development likely serves to limit competence development to conditions when DNA from closely related bacteria will be present. Cell-cell signaling influences the activity of integrative and conjugative element (ICEBs1) in two ways. First, signaling via host-encoded regulators indicates a high population density and the proximity of potential mating partners. Second, the signaling pentapeptide PhrI is produced by ICEBs1-containing cells and is used to inhibit transfer to potential partners that already contain the element. B. subtilis competence and sporulation are also regulated by two regulators, ComK and Spo0A, that are parts of multiple autoregulatory loops. They involve both positive and negative feedback regulation and help establish and maintain stable subpopulations of cells that exhibit specific patterns of gene expression and follow specific developmental fates.
Regulation of the transcription factor Spo0A by the phosphorelay and extracellular peptide signaling. The response regulator and transcription factor Spo0A is activated by a phosphorelay that transfers phosphate from histidine protein kinases, KinA to KinE, to the response regulator Spo0F, then to Spo0B, and finally to Spo0A. Spo0A~P directly activates (→) transcription of some genes and represses (⊣) transcription of others. The phosphatases RapA, RapB, RapE, and RapH promote dephosphorylation of Spo0F~P, thereby inhibiting the activation of Spo0A. The activity of RapA, RapB, RapE, and RapH is inhibited by the PhrA, PhrC (CSF), PhrE, and PhrH pentapeptides, respectively. ComA~P binds to sites upstream from rapA, C, E, and F to activate transcription.
Multiple rap and phr genes in B. subtilis. The 11 rap and 8 phr genes in the B. subtilis genome are indicated. All eight phr genes are downstream from and cotranscribed with a cognate rap. Six of the eight phr genes are known to be transcribed from at least one promoter that is sigma-H-dependent and internal to the cognate rap. The rapA, C, E, and F operons are activated by ComA~P.
Complex regulation, including autoregulatory loops, involving the transcription factor ComA and multiple extracellular signaling peptides. Stimulatory effects are indicated with arrows (→). Inhibitory effects are indicated by lines with bars (⊣). (A) ComA is activated by extracellular peptide signaling. The cell is represented by the contents of the large rectangular box with components that are intracellular, extracellular, or in the membrane. Numbers are for descriptive purposes and do not necessarily indicate sequential events. (a) The ComX pheromone is encoded together with a protein required for its production, ComQ, and the two-component signal transduction system, ComP-ComA, that regulates competence development and a quorum response. In addition to the promoter upstream from comQ, there are likely to be minor promoters (not shown) internal to the operon. (b) After transcription and translation, a precursor to ComX pheromone (pre-ComX) is modified by ComQ and exported (c) by an unknown mechanism. (d) ComX interacts on the cell surface with the membrane receptor-histidine kinase ComP and stimulates auto-kinase activity of ComP. (e) Phosphate is transferred from ComP~P to ComA. (f) ComA~P activates expression of several genes, including comS, the only ComA-target that is required for competence development. The activity of ComA~P is antagonized by the indicated Rap proteins in the absence of sufficient concentrations of the cognate Phr pentapeptides. It is currently unclear whether RapG, RapH, and RapK affect ComA directly or indirectly. (g) PhrC, PhrF, PhrG, PhrH, and PhrK pentapeptides are encoded in operons with their cognate Rap proteins. (h) The pre-Phr peptides are exported and processed through an unknown mechanism. (i) Mature Phr pentapeptides are imported into the cell through the oligopeptide permease (Opp; a. k. a., Spo0K). (j) PhrC, PhrF, PhrG, PhrH, and PhrK stimulate ComA-dependent gene expression by antagonizing the activities of their cognate Rap proteins. (B) Complex regulation of ComA and Spo0A by multiple signals, including several extracellular peptides, allows for signal integration in the control of gene expression and many autoregulatory loops. Transcription of the rap/phr genes is regulated by a variety of different proteins. The activities of these proteins are controlled by a variety of physiological signals, some of which are described in more detail in the text. Spo0A is regulated indirectly by the Rap proteins via the effects of the Raps on Spo0F. This diagram is an oversimplification of the regulatory circuits, and regulation of ComK by the Spo0A and ComA pathways is shown in Fig. 4 .
Complex regulation of the transcription factor ComK. ComK is the master transcriptional regulator of competence development. Both its stability (A) and transcription (B) are regulated by extracellular peptide signaling. For simplicity in the figure, genes (e. g., comK) and proteins (e. g., ComK) are not distinguished. (A) Control of the stability of ComK. (a) At low cell density, MecA binds to ComK and targets it for degradation by the ClpCP protease complex. (b) As cell density increases, ComA is activated by quorum sensing, and activates transcription of comS. (c) ComS disrupts the complex between ComK and MecA, liberating ComK. (d) ComK is free to activate transcription of genes that are required for competence. (e) ComS and MecA are degraded by ClpCP. (B) Control of transcription of comK. ComK activates its own expression. Its activity is controlled by quorum sensing via ComA and ComS (see above). Several other proteins also regulate comK transcription, including Spo0A and DegU, whose activities are also regulated by cell-cell signaling. The regulatory network that controls comK transcription is described more fully in the text.
The integrative and conjugative element ICEBs1 and its regulation by peptide signaling. (A) The genetic map of ICEBs1. Open reading frame and direction of transcription are indicated by thick black arrows. The ends of the element are marked by 60-bp direct repeats (black rectangles at the ends). Genes whose functions are known experimentally include rapI and phrI, toward the right end of the element; int, encoding integrase that is needed for site-specific integration and excision; immA, encoding an antirepressor; immR, encoding a transcriptional repressor; and xis, encoding excisionase. The known promoters are indicated by lines with arrows above the genes. ImmR represses transcription from the promoter immediately upstream of xis, and both activates and represses its own promoter, which also drives transcription of immA and int. The ydc and ydd genes in the central part of the element are coregulated with xis and encode most of the machinery needed for conjugal transfer. (B) Regulation of transcription of ICEBs1 by peptide signaling and the SOS response. Expression of the ICEBs1 excisionase (xis) and conjugation genes is repressed by ImmR. ImmA appears to antagonize the activity of ImmR, thereby stimulating expression of the excisionase and conjugation genes and causing excision and transfer. The activity of ImmA is stimulated by RapI. However, rapI and phrI are not significantly expressed at low cell densities and in the presence of abundant nutrients due to repression by AbrB. When cells are at high cell density and starved, Spo0A~P accumulates and inhibits abrB, leading to increased transcription of rapI and phrI. phrI is also transcribed by RNA polymerase containing sigma-H, an alternative sigma factor whose activity increases as cells enter stationary phase. The activity of RapI is inhibited by the PhrI pentapeptide, thereby inhibiting excision and transfer of ICEBs1. The concentration of the PhrI pentapeptide (encoded in ICEBs1) reflects the concentration of surrounding cells that contain ICEBs1. ImmA is also activated by RecA under conditions that induce the SOS response. The RecA and RapI pathways are independent of each other.