Pentose Metabolism in Rhizobium leguminosarum MNF300 and in Cowpea Rhizobium NGR234

Abstract

L-Arabinose is broken down by Rhizobium leguminosarum MNF300 via 2-oxoglutarate semialdehyde. Enzyme activities in cells grown on succinate, mannitol or arabinose indicated much greater modulation of arabinonate dehydratase, 2-keto-3-deoxyarabinonate dehydratase and 2-oxoglutarate semialdehyde dehydrogenase than of arabinose dehydrogenase or of arabinono-y-lactonase. In cowpea Rhizobium NGR234, all the enzymes of L-arabinose metabolism except L-arabinono-y-lactonase were inducible. Assays for such enzymes in snake bean bacteroids indicated that L-arabinose did not reach the bacteroids in large quantities. The Tn5-induced mutant MNF3045 of R . legurninosarum was unable to grow on L-arabinose and accumulated L-arabinono-y-lactone and L-arabinonate. Product accumulation and enzyme assays suggested that this mutant was defective in L-arabinonate dehydratase. It nodulated peas and the nodules fixed N2, indicating that the supply of L-arabinose is not essential for bacteroid function. Another Tn5-induced mutant of R . leguminosarum, MNF3041, lacked ribokinase and was unable to grow on D-ribose; this mutant was also able to nodulate peas and fix N,. I N T R O D U C T I O NRhizobia can efficiently utilize a variety of pentoses for growth. It is therefore possible that the supply of such compounds by host legumes may be important to nodule formation or function. One approach to the identification of compounds which are important to symbiotic N2 fixation in the legume nodule has been the study of rhizobial mutants blocked in particular steps in catabolism. A mutant of Rhizobium meliloti unable to grow on L-arabinose was identified as defective in the synthesis of 2-oxoglutarate dehydrogenase (Duncan & Fraenkel, 1979); it continued, however, to degrade L-arabinose at the same rate as the wild-type strain. A similar Tn5-induced mutant also metabolized L-arabinose without growth, and either nodulated later than the wild-type or formed no nodules at all (Duncan, 1981). Mutants of R . meliloti which were unable to grow on ribose or on xylose were normal in both nodulation and N2 fixation, and still possessed the capacity to degrade these pentoses (Duncan, 1981). While these data suggest that metabolic intermediates were accumulating, none was identified.The routes of pentose utilization in rhizobia are not well defined. In Brudyrhizobium juponicum, L-arabinose appears to be degraded to L-2-keto-3-deoxyarabinonate (KDA), which is then cleaved to pyruvate and glycolaldehyde (Fig. 1) via an aldolase reaction (Pedrosa & Zancan, 1974). While pyruvate and glycolaldehyde were identified as products of 2-keto-3-deoxyarabinonate, the alternative route, dehydration to 2-oxoglutarate semialdehyde, was not explored. It was subsequently reported (Duncan, 1979) that B. japonicum 61A76 and cowpea Bradyrhizobium 32H 1 not only possessed KDA aldolase but also 2-oxoglutarate semialdehyde dehydrogenase. The presence of this enzyme suggests that KDA dehydratase may also have Abbreviation: KDA, 2-keto-3-deoxyarabinonate. 0001-3402 0 1986 SGM

Sinorhizobium meliloti pSymB carries genes necessary for arabinose transport and catabolism

“…Based on the biochemically elucidated arabinose catabolism pathway in S. meliloti and other fast-growing Rhizobium species (Dilworth et al, 1986;Duncan & Fraenkel, 1979;Stowers, 1985), arabinose is likely catabolized to 2-oxoglutarate via five enzymic steps: L-arabinose 1-dehydrogenase, L-arabinonolactonase, L-arabinonate dehydratase, 2-keto-3-deoxy-L-arabinonate dehydratase and 2-oxoglutarate semialdehyde dehydrogenase. Based on the results of database queries, we hypothesize that araE encodes 2-oxoglutarate semialdehyde dehydrogenase and araF encodes L-arabinonate dehydratase, while araABC are involved in arabinose transport.…”

“…In both groups, the first two enzymic reactions are conserved. Following the initial reactions, the pathways diverge, producing pyruvate and glycoaldehyde in the slow growers, and 2-oxoglutarate in the fast growers (Dilworth et al, 1986;Duncan & Fraenkel, 1979;Duncan, 1979;Pedrosa & Zancan, 1974;Stowers, 1985). As S. meliloti is a member of the fast-growing rhizobia, arabinose is catabolized to 2-oxoglutarate (Duncan & Fraenkel, 1979).…”

Addition of Water to C=C Bonds

“…The glycolytic enzyme phosphoenolpyruvate (PEP) hydratase (enolase, E. C. 4.2.1.11) catalyzes the addition of water to 2-phospho-~-glycerate (26). The enzyme from E. c0li1~~1 also accepts 3-phospho-~-erythronate (28) and thereby forms phosphoenol-4-deoxy-3-tetrulosonate (29 in Scheme 11.5-4).…”

Regulation of the mdh-sucCDAB operon in Rhizobium leguminosarum

“…There are only small changes in activity for most substrates relative to succinate, with the exception of arabinose, which resulted in a 2.5-fold increase. Arabinose is converted directly into K-ketoglutarate in R. leguminosarum [14], suggesting that there may be a signi¢cant role for K-ketoglutarate levels in regulating the mdh promoter. This might also contribute to the 7-fold increase in activity of the mdh promoter seen in suc mutants, since these strains produce large quantities of K-ketoglutarate because they are blocked for breakdown of this substrate.…”