FIG. 1.

Proposed pathway for d-xylose metabolism in C. crescentus. The reactions shown are based on biochemically confirmed degradation pathways for d-xylose metabolism in pseudomonads (7, 27). Both d-xylose and l-arabinose produce 2-keto-3-deoxy-pentonate. In the Dahms pathway (7), this compound is converted by an aldolase to pyruvate and glycoaldehyde. In an alternative reaction first demonstrated by Weimberg (27) and confirmed by Watanabe et al. (25, 26), for l-arabinose degradation in A. brasilense, a dehydratase produces α-ketoglutarate semialdehyde, which is then oxidized to α-ketoglutarate. The genes identified (through mutation) in this work as necessary for growth on d-xylose (Table 1) and the enzymes they encode are shown beside the appropriate reaction. The Entner-Doudoroff pathway and alternative reactions used in gluconeogenesis are shown at the upper right. TCA cycle reactions (in the box on the lower left, not shown in detail) are expected to be necessary for both d-xylose and d-glucose metabolism; genes encoding these enzymes were probably not found in this screen because they are also necessary for growth on PYE medium.