Ammonia is the Preferred Nitrogen Source in Several Rhizobia

SUMMARY: When presented with an equimolar mixture of ammonia and L-glutamate, batch cultures of Rhizobium leguminosarum MNF3841 and ‘R. trifolii’ (R. leguminosarum biovar trifolii) TA1 used ammonia at a significantly faster rate than L-glutamate. The cowpea Rhizobium strain NGR234, however, used L-glutamate at a marginally faster rate than ammonia. R. leguminosarum MNF3841 also grew faster on ammonia than on L-glutamate as the nitrogen source.

Chemostat cultures of R. leguminosarum MNF3841 limited for phosphate did not release excess ammonia when grown on mannitol/L-glutamate, showing that L-glutamate catabolism was tightly regulated to meet the cells' nitrogen requirement. Further, the rate of consumption of ammonia was similar to that for L-glutamate when either was supplied as the sole nitrogen source. However, with L-histidine or L-alanine as a nitrogen source, large quantities of excess ammonia were released. When chemostat cultures of R. leguminosarum MNF3841 were supplied with an equimolar mixture of ammonia and L-glutamate, 89-100% of the nitrogen consumed was ammonia. Similarly, with mixtures of L-glutamate/L-histidine or L-glutamate/L-alanine, almost no L-glutamate was consumed, a result attributable to the release of excess ammonia from either L-histidine or L-alanine. The use of 14C-labelled fructose or L-glutamate showed that the intra- and extracellular L-glutamate pools were isolated, indicating that the ammonia preference must be exerted by a restriction in L-glutamate transport. L-Glutamate transport rates were low in chemostats containing L-glutamate/NH4Cl, an effect attributable in part to a significant repression of synthesis of the L-glutamate transport system by ammonia.