Moonlighting in metabolism: bifunctional enzymes control nitrogen metabolism in Bacillus subtilis

SUMMARYGlutamate is an essential building block and the most important amino group donor in the cell. The reactions involved in synthesis and degradation link carbon to nitrogen metabolism. The synthesis and activity of the enzymes catalyzing these reactions must, therefore, be precisely regulated. In the Gram-positive model bacterium Bacillus subtilis, glutamate is exclusively synthesized by the combined action of the glutamine synthetase (GS) and the glutamate synthase (GOGAT). The GS catalyzes the ATP-dependent assimilation of ammonium, resulting in the formation of glutamine. Glutamine is converted together with 2-oxoglutarate by the GOGAT into glutamate, which can be used either for further assimilation of ammonium, or as a building block, or amino group donor. The glutamate dehydrogenases (GDHs) GudB and RocG are strictly devoted to glutamate degradation. In recent years, exciting new observations have been made in nitrogen metabolism in B. subtilis. The GS, GOGAT, and the GDHs are multifunctional enzymes, with the GS and GDHs acting as trigger enzymes in the control of gene expression, in addition to their enzymatic activity. The glutamate-synthesizing GOGAT acts as a counter enzyme, inactivating the major GDH GudB to prevent a futile cycle. In this review, we intend to summarize the current state of knowledge about nitrogen metabolism in B. subtilis and discuss open questions that need to be answered in the future.