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21 Bacillus subtilis Sporulation and Other Multicellular Behaviors
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This chapter focuses on Bacillus subtilis multicellularity, emphasizing the two-cell differentiation process of endospore formation and attempting to note similarities to Myxococcus xanthus. While cell growth, division, motility, and chemotaxis clearly play roles in forming bioconvection patterns, complex colonies, and macrofibers, these multicellular phenomena have not yet been subjected to systematic genetic analysis. In contrast, recently discovered multicellular behaviors of biofilm formation and swarming motility are rapidly being elucidated by genetic and genomic approaches. The most studied and best understood multicellular behaviors of B. subtilis are the development of genetic competence (the ability to take up exogenous DNA) and sporulation. The chapter summarizes the understanding of how morphogenesis and intercellular signaling control the activity of cell-specific s factors, focusing on recent progress and attempting to identify questions that remain. It also reviews the results of genomic approaches to characterize the regulon of each cell-specific s factor and the functions of some of the gene products. The best-characterized multicellular behaviors of B. subtilis, sporulation and the development of competence to take up exogenous DNA, are regulated by extracellular peptide signaling, analogous to M. xanthus A-signaling. Proteolysis is already known to play roles in A-, B-, and C-signaling during M. xanthus development, and it seems likely that many more roles will be uncovered, based on studies of B. subtilis. Just as these studies have provided a host of paradigms, so too will continued investigation of the myxobacteria and their neighbors continue to yield novel insights of medical, economic, and environmental benefit.
Morphological changes and the approximate time and location at which different σ factors become active during B. subtilis sporulation. See the text for explanation. Reproduced from Kroos and Maddock, 2003 , with permission.
Regulation of σF activity. (A) Illustration of the reactions that result in σF being activated in the forespore. Critical determinants are thought to be the concentrations of SpoIIE and SpoIIAA-PO4; a long-lived SpoIIAA-SpoIIAB-ADP complex (a SpoIIAA/SpoIIAB “sink”); and instability of free SpoIIAB combined with transient genetic asymmetry so that both copies of the spoIIAB gene are in the mother cell. (B) Schematic illustration of the effects of the sporulation division on the regulators of σF. The SpoIIAA-PO4 protein (open pentagons) is presumed to be evenly distributed throughout the cytoplasm so that most is present in the mother cell. The SpoIIE protein (filled diamonds) is associated with the septum, and most of it may be associated with SpoIIAA-PO4, such that the SpoIIE: SpoIIAA-PO4 complex is distributed equally between mother cell and forespore. Only the origin-proximal 30% of a chromosome is present in the forespore when first formed. As a consequence both copies of the spoIIAB gene are present in the mother cell (MC), and degradation disproportionally reduces the SpoIIAB concentration in the forespore.
Signaling pathway governing pro-σE processing. The upper part shows the sporangium after the asymmetric septum forms. The lower part shows components of the signaling pathway. SpoIIR made in the forespore is translocated across the forespore membrane of the septum and activates SpoIIGA to cleave pro-σE.
Organization and transcription of the spoIIG and sigG operons. Indicated are the two genes of the spoIIG operon, spoIIGA, the protease believed to process pro-σE, and sigE, the structural gene for pro-σE. Also shown is sigG, the gene encoding σG. An open reading frame located downstream from sigG, ylmA (not shown), is cotranscribed with sigG. Transcription from PspoIIG requires σA-RNAP and phosphorylated Spo0A. Some of the PspoIIG transcript reads through the sigG operon. PsigG is used weakly by σF RNAP and more strongly by σG RNAP.
Model for the regulation of pro-σk processing. (A) The upper part shows the sporangium after engulfment of the forespore within the mother cell. Black ovals represent a protein complex that bridges the two membranes surrounding the forespore. The lower part shows an expanded view of the protein complex that bridges from the inner forespore membrane (IFM) to the outer forespore membrane (OFM). SpoIVB is a serine protease made initially under σF control in the forespore and believed to be translocated across the IFM. A low level of SpoIVB is sufficient to cause proteolysis of the SpoIIQ extracellular domain. σG RNAP boosts the level of SpoIVB and it cleaves the C-terminal extracellular domain of SpoIVFA. An unknown protein (Protein X) is proposed to localize the pro-σK processing machinery to the SpoIIQ-SpoIIIAH complex. (B) Loss of SpoIVFA renders BofA susceptible to cleavage by CtpB, a serine protease made under σE control in the mother cell and under σG control in the forespore. CtpB is believed to be translocated into the space between the two membranes, where it targets a short C-terminal extracellular domain of BofA. (C) Loss of BofA allows SpoIVFB to cleave pro-σK, releasing active σK into the mother cell. See the text for references.