Establishment of Oxidative d-Xylose Metabolism in Pseudomonas putida S12

The oxidative d-xylose catabolic pathway of Caulobacter crescentus, encoded by the xylXABCD operon, was expressed in the gram-negative bacterium Pseudomonas putida S12. This engineered transformant strain was able to grow on d-xylose as a sole carbon source with a biomass yield of 53% (based on g [dry weight] g d-xylose⁻¹) and a maximum growth rate of 0.21 h⁻¹. Remarkably, most of the genes of the xylXABCD operon appeared to be dispensable for growth on d-xylose. Only the xylD gene, encoding d-xylonate dehydratase, proved to be essential for establishing an oxidative d-xylose catabolic pathway in P. putida S12. The growth performance on d-xylose was, however, greatly improved by coexpression of xylXA, encoding 2-keto-3-deoxy-d-xylonate dehydratase and α-ketoglutaric semialdehyde dehydrogenase, respectively. The endogenous periplasmic glucose dehydrogenase (Gcd) of P. putida S12 was found to play a key role in efficient oxidative d-xylose utilization. Gcd activity not only contributes to d-xylose oxidation but also prevents the intracellular accumulation of toxic catabolic intermediates which delays or even eliminates growth on d-xylose.The requirement for renewable alternatives to replace oil-based chemicals and fuels necessitates development of novel technologies. Lignocellulose provides a promising alternative feedstock. However, since the pentose sugar fraction may account for up to 25% of lignocellulosic biomass (12), it is essential that this fraction is utilized efficiently to obtain cost-effective biochemical production. In a previous study, the solvent-tolerant bacterium Pseudomonas putida S12, known for its use as a platform host for the production of aromatic compounds (15, 16, 19, 22), was engineered to use d-xylose as a sole carbon source. This was achieved by introducing genes encoding the phosphorylative d-xylose metabolic pathway of Escherichia coli, followed by laboratory evolution (14). Prior to evolutionary improvement, extensive oxidation of d-xylose to d-xylonate occurred, resulting in a very low biomass-for-substrate yield as d-xylonate is a metabolic dead-end product in P. putida. The evolution approach resulted in elimination of the activity of periplasmic glucose dehydrogenase (Gcd), the enzyme responsible for d-xylose oxidation, which turned out to be a critical step in optimizing phosphorylative d-xylose utilization in P. putida S12.Instead of prevention of endogenous oxidation of d-xylose, this oxidation may be used to our advantage when it is combined with an oxidative d-xylose metabolic pathway, such as the pathways described for several Pseudomonas species, Caulobacter crescentus, and Haloarcula marismortui (7, 11, 18, 20). In these pathways, d-xylonate is dehydrated to 2-keto-3-deoxy-d-xylonate. This intermediate either can be cleaved into pyruvate and glycolaldehyde (7) or is further dehydrated to α-ketoglutaric semialdehyde (α-KGSA). In the final step of the latter pathway, α-KGSA is oxidized to the tricarboxylic acid (TCA) cycle intermediate α-ketoglutarate (18, 20).In addition to Gcd (PP1444), some of the enzymes required for oxidative d-xylose metabolism are expected to be endogenous in P. putida S12. Transport of d-xylonate into the cytoplasm likely occurs through the gluconate transporter (encoded by gntP [PP3417]). The enzyme catalyzing the final step of the pathway, α-KGSA dehydrogenase, is also likely to be present (presumably PP1256 and/or PP3602) because of the requirement for metabolism of 4-hydroxyproline (1), a compound that is efficiently utilized by P. putida S12. In view of these properties, the most obvious approach for constructing d-xylose-utilizing P. putida S12 is reconstruction of a complete oxidative d-xylose metabolic pathway by introducing the parts of such a pathway that complement the endogenous activities. Recently, the genetic information for one such oxidative d-xylose pathway has become available (18), enabling the approach used in the present study, i.e., expression of the oxidative d-xylose metabolic pathway of C. crescentus in P. putida S12 and investigation of the contribution of endogenous enzyme activities.