FIG 1.
Regulation of the arabinose and xylose sugar utilization systems. Both systems are induced by their cognate sugar. Arabinose-bound AraC induces the expression of the arabinose metabolic genes (araBAD), high-affinity transporter genes (araFGH), and low-affinity transporter gene (araE) (9). In addition, arabinose-bound AraC represses its own expression. Xylose-bound XlyR induces the expression of the xylose metabolic genes (xylAB), high-affinity transporter genes (xylFGH) and, presumably, a low-affinity transport gene (xylE) (10). Xylose-bound XylR does not appear to regulate the PxylR promoter, though it may induce its expression due to transcription from the upstream PxylF promoter. In addition, the two systems are subject to transcriptional cross talk. Arabinose-bound AraC represses the expression of the xylose genes (5) and, as demonstrated in this study, xylose-bound XylR represses the expression of the arabinose genes. Furthermore, the arabinose and xylose systems are both repressed by glucose (mechanism not shown).