BIOSYNTHESIS AND EXPRESSION OF CELL-SURFACE POLYSACCHARIDES IN GRAM-NEGATIVE BACTERIA

This chapter provides an overview of the molecular mechanisms involved in synthesis and expression of cell-surface polysaccharides in Gram-negative bacteria. Biosynthesis of many cell-surface components, including polysaccharides, involves enzymes and enzyme complexes found in the cytoplasmic membrane. The peptidoglycan layer is located immediately external to the cytoplasmic membrane and this layer is required for cell shape and rigidity. Gram-negative bacteria possess a periplasm that contains a variety of proteins and enzymes, including some involved in import and export of macromolecules. Biosynthesis of bacterial cell-surface polysaccharides involves a series of sequential processes: (1) biosynthesis of activated precursors in the cytoplasm, (2) formation of repeating units, (3) polymerization of repeating units, and (d) export of polysaccharides to the cell surface. The assembly of polysaccharide repeating units and subsequent polymerization reactions occur at the cytoplasmic membrane, using precursors synthesized in the cytoplasm. Genes for biosynthesis of cell-surface polysaccharides are chromosomal and are arranged in clusters of one or more transcriptional units. The synthesis of lipopolysaccharide (LPS) may be subject to complex regulation, but on-off switching is not possible due to the essential structural requirement for the lipid A-core LPS molecule. Most bacteria use extracellular polysaccharides (EPSs) for protection, and many regulatory strategies are directed to modulating EPS synthesis in response to appropriate environmental cues. Application of genetic and biochemical approaches has facilitated detailed analysis of complex, multicomponent systems, such as those involved in synthesis of cell-surface polysaccharides. © 1993, Academic Press Limited