Effect of regulatory protein levels on utilization of starch by Bacteroides thetaiotaomicron

Abstract

Bacteroides thetaiotaomicron, a gram-negative obligate anaerobe, appears to utilize starch by first binding the polymer to its surface and then translocating it into the periplasmic space. Several genes that encode enzymes or outer membrane proteins involved in starch utilization have been identified. These have been called sus genes, for starch utilization system. Previous studies have shown that sus structural genes are regulated at the transcriptional level and their expression is induced by maltose. We report here the identification and characterization of a gene, susR, which appears to be responsible for maltose-dependent regulation of the sus structural genes. The deduced amino acid sequence of SusR protein had a helix-turn-helix motif at its carboxy-terminal end, and this region had highest sequence similarity to the corresponding regions of known transcriptional activators. A disruption in susR eliminated the expression of all known sus structural genes, as expected if susR encoded an activator of sus gene expression. The expression of susR itself was not affected by the growth substrate and was not autoregulated, suggesting that binding of SusR to maltose might be the step that activates SusR. Three susR-controlled structural genes, susA, susB, and susC, are located immediately upstream of susR. These genes are organized into two transcriptional units, one containing susA and another containing susB and susC. susA was expressed at a lower level than susBC, and susA expression was more sensitive to the gene dosage of susR than was that of the susBC operon. An unexpected finding was that increasing the number of copies of susR in B. thetaiotaomicron increased the rate of growth on starch. This effect could be due to higher levels of susA expression. Whatever the explanation, the level of SusR in the cell appears to be a limiting factor for growth on starch.

Transcriptomic Analyses of Xylan Degradation by Prevotella bryantii and Insights into Energy Acquisition by Xylanolytic Bacteroidetes

“…2). It has been proposed that the Sus system, initially identified by Salyers and co-workers (37,39), represents a paradigm for oligo-and polysaccharide utilization by Bacteroidetes (40), and the core xylan utilization cluster identified in this study provides further support for this prediction.…”

Bacteroides: the Good, the Bad, and the Nitty-Gritty

“…At sites of infection, B. fragilis may utilize host cell surface glycoproteins and glycolipids as a nutrient source; these may include simple sugars such as galactose and mannose and more complex compounds (e.g., N-acetyl-D-glucosamine [NAG]) and N-acetylneuraminic acids). Indeed, the largest paralogous group of proteins in B. thetaiotaomicron are those involved in oligo-and polysaccharide uptake and degradation (2,3,58,59,72,73,156,206,207,229,247,248,276), capsular biosynthesis, and environmental sensing/signal transduction/DNA mobilization (306). The authors of the B. thetaiotaomicron genome sequence publication suggest that these expansions "reveal strategies used by B. thetaiotaomicron to survive and to dominate in the densely populated intestinal system" (306).…”

A hybrid two-component system protein of a prominent human gut symbiont couples glycan sensing in vivo to carbohydrate metabolism

“…BT3172, in turn, induces expression of the ␣-mannosidases that degrade these mannosides. Glycosidic linkage-dependent regulation of gene expression is known to occur in B. thetaiotaomicron's starch utilization system (Sus) operon, which is induced by maltose and higher order starches, but not by glucose alone (14). Linkagespecific activation is consistent with the idea that at least some B. thetaiotaomicron HTCS incorporate all of their input and output domains into one polypeptide so that they can sense very specific nutrients in their habitat and mobilize appropriate cellular transcriptional and metabolic responses without the need for signal amplification.…”