Disulfide bonds formed between pairs of cysteines are important features of the structure of many proteins. Elaborate electron transfer pathways have evolved Escherichia coli to promote the formation of these covalent bonds and to ensure that the correct pairs of cysteines are joined in the final folded protein. These transfers of electrons consist, in the main, of cascades of disulfide bond formation or reduction steps between a series of proteins (DsbA, DsbB, DsbC, and DsbD). A surprising variety of mechanisms and protein structures are involved in carrying out these steps.

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The cytoplasmic membrane protein DsbD transfers electrons from the cytoplasm to the periplasm of E. coli, where its reducing power is used to maintain cysteines in certain proteins in the reduced state. We split DsbD into three structural domains, each containing two essential cysteines. Remarkably, when coexpressed, these truncated proteins restore DsbD function. Utilizing this three piece system, we were able to determine a pathway of the electrons through DsbD. Our findings strongly suggest that the pathway is based on a series of multistep redox reactions that include direct interactions between thioredoxin and DsbD, and between DsbD and its periplasmic substrates. A thioredoxin-fold domain in DsbD appears to have the novel role of intramolecular electron shuttle.

Xanthan gum is a complex exopolysaccharide produced by the plant-pathogenic bacterium Xanthomonas campestris pv. campestris. It consists of D-glucosyl, D-mannosyl, and D-glucuronyl acid residues in a molar ratio of 2:2:1 and variable proportions of O-acetyl and pyruvyl residues. Because of its physical properties, it is widely used as a thickener or viscosifier in both food and non-food industries. Xanthan gum is also used as a stabilizer for a wide variety of suspensions, emulsions, and foams. This article outlines aspects of the biochemical assembly and genetic loci involved in its biosynthesis, including the synthesis of the sugar nucleotide substrates, the building and decoration of the pentasaccharide subunit, and the polymerization and secretion of the polymer. An overview of the applications and industrial production of xanthan is also covered.