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PhD Completion – Chris Greening

Posted by on 11 March 2014 | Comments

Congratulations to Chris Greening, who recently completed his PhD in Gregory Cook’s lab.

Thesis title: Physiological roles of the three [NiFe]- hydrogenases of Mycobacterium smegmatis

Mycobacteria are represented by successful environmental saprophytes and notorious obligate pathogens.  While these organisms primarily derive their energy from aerobic respiration of organic carbon sources, they are capable of adapting to environmental changes such as nutrient and oxygen deprivation.  In this study, I revealed that hydrogen metabolism has a crucial role in the energetics of the model soil bacterium Mycobacterium smegmatis during growth, carbon-limitation, and oxygen-limitation.  This metabolism is mediated by three phylogenetically-distinct, differentially-expressed [NiFe]-hydrogenases.  Combining high-sensitivity gas chromatography, phenotypic profiling, and cellular assays, I revealed that two hydrogenases couple the oxidation of H2 to the reduction of O2 through the respiratory chain; this process is required for optimal cell growth and long-term survival. I further demonstrated that a third enzyme, activated during oxygen-limitation in a DosR-dependent manner, maintains redox homeostasis by coupling the reoxidation of NAD(P)H to the evolution of H2. This H2 can be recycled when aerobic or anaerobic acceptors become available as a mechanism for sustaining proton-motive force. Crucially, this work confirms the principal sink in the global biogeochemical cycle of H2. It additionally reveals roles for fermentation in mycobacteria and hydrogen metabolism in long-term survival for the first time.