Figure 1.

Organization of the starch and levan PULs in Bacteroides thetaiotaomicron and model of levan utilization. A. Genomic organization of B. thetaiotaomicron's prototypic Sus locus (top) and levan utilization locus (bottom). Genes of similar function are coded by color; intervening unrelated genes are white. B. Based on our studies and previous genetic and biochemical studies focused on other PULs of B. thetaiotaomicron,¹–⁵,⁷,⁸ we have developed a working model of β2-6 fructan utilization in B. thetaiotaomicron. In this model, levan is bound by the SusD homolog (D), BT1762 and the susE-positioned gene product, BT1761, on the surface of the bacterium. The polysaccharide is then cleaved into oligosaccharides by the extracellular endo-acting levanase, BT1760. Oligosaccharides are actively imported by the SusC homolog (C), BT1763 (a TonB dependent porin). In the periplasm, exo-acting glycoside hydrolases, BT1759 and BT3082, liberate monosaccharide fructose (green pentagons) from the oligosaccharides. Free fructose binds to the periplasmic sensor domain of the homodimeric HTCS, BT1754 (inset red box shows structure of BT1754 periplasmic domain homodimer bound to fructose), activating the regulatory protein and resulting in upregulation of the PUL. The inner membrane monosaccharide importer, BT1758, transports periplasmic fructose into the cell where it is shunted into the glycolytic pathway upon phosphorylation by the PUL-encoded fructokinase, BT1757. Some transport across the inner membrane of the disaccharide products of fructan degradation (sucrose and levanbiose) also occurs and these are broken down to their constituent monosaccharides by the putative intracellular fructosidase, BT1765 (not shown). Prior to activation, low levels of all components of the PUL are expressed constitutively such that B. thetaiotaomicron is always in a ‘prepared’ state.