A promiscuous Bcd amino acid dehydrogenase promotes biofilm development in Bacillus subtilis

Glutamate dehydrogenase (GDH) resides at the crossroads of nitrogen and carbon metabolism, catalyzing the reversible conversion of L-glutamate to α-ketoglutarate and ammonium. GDH paralogs are ubiquitous across most species, presumably enabling functional specialization and genetic compensation in response to diverse conditions. Staphylococcus aureus harbors a single housekeeping GDH (GudB), whereas Bacillus subtilis encodes both a major and a minor GDH, GudB and RocG, respectively. In an unsuccessful attempt to identify an alternative GDH in S. aureus, we serendipitously discovered previously unrecognized GDH activity in two metabolic enzymes of B. subtilis. The hexameric Val/Leu/Ile dehydrogenase Bcd (formerly YqiT) catabolizes branched-chain amino acids and to a lesser extent glutamate using NAD⁺ as a cofactor. Removal of gudB and rocG unmasks the dual NAD(P)⁺-dependent GDH activity of RocA, which otherwise functions as a 3-hydroxy-1-pyrroline-5-carboxylate dehydrogenase. Bcd homologs are prevalent in free-living and obligate bacteria but are absent in most, if not all, staphylococci. Despite low sequence homology, Bcd structurally resembles the GudB/RocG family and can functionally compensate for the loss of GudB in S. aureus. Bcd is essential for the full maturation of biofilms. B. subtilis lacking GDHs exhibits severe impairments in rugose architecture and colony expansion of biofilms. This study underscores the importance of metabolic redundancy and highlights the critical role of substrate promiscuity in GDHs during biofilm development.