Ammonia assimilation and glutamate formation in Caulobacter crescentus

In the dimorphic bacterium Caulobacter crescentus, ammonia assimilation occurs only via the combined action of the enzymes glutamine synthetase and glutamate synthase. Mutants auxotrophic for glutamate lacked glutamate synthase activity, and the mutations leading to the glutamate auxotrophy appeared to lie at two distinct genetic loci. Both glutamate synthase and glutamine synthetase activities were subject to regulation by repression. Glutamate synthase activity was highest in cultures grown in minimal medium with ammonia as sole nitrogen source and was about fivefold lower in rich broth. Glutamine synthetase activity was highest in cells grown with growth-rate-limiting amounts of ammonia as nitrogen source and was about fourfold lower in rich broth. In addition, glutamine synthetase activity appeared to be regulated by an adenylylation system like that described for Escherichia coli.