Regulation of a catabolic enzyme whose substrate is always available.

Certain hydrogenases are capable of converting the trace amounts of molecular hydrogen constantly present in Earth's atmosphere to supply cells with a little energy during periods of starvation. Recently, Kropp and colleagues investigated the regulation of one of these "high-affinity" hydrogenases in response to different growth conditions for the ubiquitous soil bacterium Mycobacterium smegmatis (A. Kropp, J. D. Archer, M. Jespersen, T. D. Watts, et al., mSystems 11:e01678-25, 2026, https://doi.org/10.1128/msystems.01678-25). A mutation in the gylR gene, whose product acts as a positive transcriptional regulator of the genes involved in glycerol catabolism, causes the corresponding mutant strain to grow very slowly on glycerol, while the activity of the high-affinity hydrogenase reached levels more than 50 times higher than those of the wild-type strain of M. smegmatis. The results of this study suggest that the synthesis of the hydrogenase is subject to a regulatory mechanism similar to carbon catabolite repression, which is entirely consistent with the cellular function of this enzyme.