Abstract
The soil bacterium Mycobacterium smegmatis is able to scavenge the trace concentrations of H-2 present in the atmosphere, but the physiological function and importance of this activity is not understood. We have shown that atmospheric H-2 oxidation in this organism depends on two phylogenetically and kinetically distinct high-affinity hydrogenases, Hyd1 (MSMEG_2262-2263) and Hyd2 (MSMEG_2720-2719). In this study, we explored the effect of deleting Hyd2 on cellular physiology by comparing the viability, energetics, transcriptomes, and metabolomes of wild-type vs. Delta hyd2 cells. The long-term survival of the Delta hyd2 mutant was significantly reduced compared to the wild-type. The mutant additionally grew less efficiently in a range of conditions, most notably during metabolism of short-chain fatty acids; there was a twofold reduction in growth rate and growth yield of the Delta hyd2 strain when acetate served as the sole carbon source. Hyd1 compensated for loss of Hyd2 when cells were grown in a high H-2 atmosphere. Analysis of cellular parameters showed that Hyd2 was not necessary to generate the membrane potential, maintain intracellular pH homeostasis, or sustain redox balance. However, microarray analysis indicated that Delta hyd2 cells were starved for reductant and compensated by rewiring central metabolism; transcripts encoding proteins responsible for oxidative decarboxylation pathways, the urea cycle, and ABC transporter-mediated import were significantly more abundant in the Delta hyd2 mutant. Metabolome profiling consistently revealed an increase in intracellular amino acids in the Delta hyd2 mutant. We propose that atmospheric H2 oxidation has two major roles in mycobacterial cells: to generate reductant during mixotrophic growth and to sustain the respiratory chain during dormancy.