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13 Composition, Structure, and Function of the Myxococcus xanthus Cell Envelope
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This chapter covers in detail the polysaccharide-containing components of the Myxococcus xanthus cell envelope including the peptidoglycan (PG), lipopolysaccharide (LPS), extracellular matrix (ECM), and capsular exopolysaccharide (EPS). First, the PG was associated with substantial amounts of glycine, serine, and glucose. Second, the vegetative cell wall PG was suggested to be discontinuous in that whole sacculi were not isolated and trypsin and sodium dodecyl sulfate were able to completely disassociate the PG. At least two factors regulate MBHA accumulation. First, mbhA transcription, which is σ-54 dependent, increases during development. Second, the stability of the mbhA mRNA is increased during development. Interestingly, all of the LPS mutants identified by the authors' two laboratories mapped to three loci: two LPS O-antigen loci and one LPS core locus. The carbohydrate composition of wild-type LPS consists of glucose, mannose, rhamnose, arabinose, xylose, galactosamine, glucosamine, KDO (2-keto-3-deoxyoctulosonic acid), and 3-O-methylpentose and 6-O-methylgalactosamine. Signal perception by the Dif pathway involves Tfp. First, Tfp was found to be required for EPS production in M. xanthus. Second, Dif proteins function downstream of Tfp as demonstrated by genetic epistasis tests. Finally, Tfp do not appear to function as either exogenous or endogenous signals for the Dif pathway. Preliminary biochemical studies indicated that M. xanthus EPS contains at least five monosaccharides: galactose, glucosamine, glucose, rhamnose, and xylose. Studies of M. xanthus pili focus on their involvement in S-motility.
Chain-like strand in the M. xanthus periplasm and the structural model. (a) Single strand fragment associated with a membrane fragment (MF). The ring elements (arrowheads) stand oblique to the plane of the carbon support. Inset: ring elements (circle) and central elongated elements (dots) are shown at higher magnification; arrow indicates the spoke-like central mass. Bar, 50 nm (inset, 30 nm). (b). Three-dimensional model of the location of strands within the cell wall of M. xanthus. These strands are proposed to be inserted between the flexible peptidoglycan sacculus and the outer membrane. RE, ring element, framed; EE, elongated element; CM, cytoplasmic membrane; OM, outer membrane; cema, central mass; pm, peripheral mass. Adapted from Freese et al., 1997.
ECM visualized by different EM techniques. (A and B) Transmission EM micrographs prepared either by spray-freezing freeze substitution (A) or by staining with lanthanum and fixation with glutaraldehyde (B). (C and D) SEM images at different magnifications. Samples in panels C and D were fixed with glutaraldehyde and coated with platinum. Fig. 2A from Kim et al., 1999; Fig. 2B from Merroun et al., 2003; Fig. 2C and D from Behmlander and Dworkin, 1991.
Model depicting the regulation of EPS production in M. xanthus by Tfp and the Dif pathway. Demonstrated interactions are indicated by solid lines, and proposed interactions are indicated by dashed lines. Arrows and bars indicate positive and negative regulation, respectively. See the text for details of the model. Adapted from Black et al., 2006.