Evidence for a Low Affinity but High Velocity Aspartate Transport System Needed for Rapid Growth of Bacillus subtilis on Aspartate as Sole Carbon Source

Abstract

Mutants (aspT) of Bacillus subtilis which lack the high affinity transport of L-aspartate and L-glutamate have been isolated by their resistance to DL-threo-β-hydroxyaspartate. They transport low concentrations (100 μM) of all three amino acids at a greatly reduced rate but they can still grow at the normal rate on high concentrations of aspartate (25 mM) as sole carbon source. The aspT mutation has been mapped between argC and glpK, glpD. In glucose/citrate medium, an aspB mutant, auxotrophic for aspartate, requires much lower aspartate concentrations for growth than an aspB aspT double mutant. These results demonstrate that a low affinity aspartate transport system is still present in aspT mutant strains. This was also shown by the fact that both aspH and aspH aspT mutants, which carry the gene (aspH) for high aspartase production, grew at the same rate in media containing high concentrations (25 mM) of L-aspartate as sole carbon source. The high affinity aspartate transport system (K m = 67 μM) alone can satisfy the growth requirements of aspartate auxotrophs in media containing glucose or some other carbon source. However, the maximum rate (V max) of this transport system is too low to allow rapid growth on aspartate as sole carbon source. For such rapid uptake the low affinity aspartate transport system is needed. Membrane vesicles, energized by glycerophosphate, exhibit only the active high affinity transport which is saturated at about 100 μM-L-aspartate (K m = 66 μM). Apparently, the proton motive force, which energizes the high affinity aspartate (and glutamate) transport, is not (directly) required for the low affinity aspartate transport.