Characterization of Bacillus subtilis mutants with carbon source-independent glutamate biosynthesis

Bacillus subtilis synthesizes glutamate from 2-oxoglutarate and glutamine using the glutamate synthase, encoded by the gltAB operon. Glutamate degradation involves the catabolic glutamate dehydrogenase (GDH) RocG. Expression of both gltAB and rocG is controlled by the carbon and nitrogen sources. In the absence of glucose or other well-metabolizable carbon sources, B. subtilis is unable to grow unless provided with external glutamate. In this work, we isolated mutations that suppressed this growth defect of B. subtilis on minimal media (sgd mutants). All mutations enabled the cells to express the gltAB operon even in the absence of glucose. The mutations were all identified in the rocG gene suggesting that the catabolic GDH is essential for controlling gltAB expression in response to the availability of sugars.