Transcriptomic profiling and its implications for the H-2 production of a non-methanogen deficient in the frhAGB-encoding hydrogenase

Abstract

The F-420-reducing hydrogenase of methanogens functions in methanogenesis by providing reduced coenzyme F-420 (F420H2) as an electron donor. In non-methanogens, however, their physiological function has not been identified yet. In this study, we constructed an Delta frhA mutant, whose frhA gene encoding the hydrogenase alpha subunit was deleted, in the non-methanogenic Thermococcus onnurineus NA1 as a model organism. There was no significant difference in the formate-dependent growth between the mutant and the wild-type strains. Interestingly, the mutation in the frhA gene affected the expression of genes involved in various cellular functions such as H-2 oxidation, chemotactic signal transduction, and carbon monoxide (CO) metabolism. Among these genes, the CO oxidation gene cluster, enabling CO-dependent growth and H-2 production, showed a 2.8- to 7.0-fold upregulation by microarray-based whole transcriptome expression profiling. The levels of proteins produced by this gene cluster were also significantly increased not only under the formate condition but also under the CO condition. In a controlled bioreactor, where 100% CO was continuously fed, the Delta frhA mutant exhibited significant increases in cell growth (2.8-fold) and H-2 production (3.4-fold). These findings strongly imply that this hydrogenase is functional in non-methanogens and is related to various cellular metabolic processes through an unidentified mechanism. An understanding of the mechanism by which the frhA gene deletion affected the expression of other genes will provide insights that can be applied to the development of strategies for the enhancement of H-2 production using CO as a substrate.