ASPARTATE TRANSPORT IN RHIZOBIUM-MELILOTI

Aspartate transport in Rhizobium meliloti was found to be mediated by at least two transport systems. High rates of aspartate uptake, necess ary for growth on aspartate as a carbon source, required the dicarboxy late transport (Dct) system, which also transports succinate, fumarate and malate. The apparent K(m) for aspartate transport by this system was about 10 mM, compared to 15 muM for succinate. This difference in affinity was also apparent in competitive inhibition studies, which sh owed that succinate effectively inhibits aspartate transport. Although aspartate was not a preferred substrate, it was a very efficient indu cer of the Dct system. Both the Dct system and a second aspartate tran sport system were capable of supplying aspartate for use as a nitrogen source. The second system had a lower apparent K(m) for aspartate tra nsport (1 -5 mM), and was competitively inhibited by glutamate. This a spartate-glutamate system was regulated independently from the Dct sys tem, since it functioned in mutants lacking the Dct system regulatory genes dctB and dctD, and its induction did not coactivate the Dct syst em. Uptake kinetics in cultures growing on aspartate as nitrogen sourc e showed rapid substrate exchange between extracellular and internal a spartate. R. meliloti was shown to be able to selectively activate the two uptake systems, and also regulated its metabolism as required to utilize aspartate as either carbon or nitrogen source.