Metabolic Adaptations in a H2 Producing Heterocyst-Forming Cyanobacterium: Potentials and Implications for Biological Engineering

Nostoc punctiforme ATCC 29133 is a photoautotrophic cyanobacterium with the ability to fix atmospheric nitrogen and photoproduce hydrogen through the enzyme nitrogenase. The H2 produced is reoxidized by an uptake hydrogenase. Inactivation of the uptake hydrogenase in N. punctiforme leads to increased H2 release but unchanged rates of N2 fixation, indicating redirected metabolism. System-wide understanding of the mechanisms of this metabolic redirection was obtained using complementary quantitative proteomic approaches, at both the filament and the heterocyst level. Of the total 1070 identified and quantified proteins, 239 were differentially expressed in the uptake hydrogenase mutant (NHM5) as compared to wild type. Our results indicate that the inactivation of uptake hydrogenase in N. punctiforme changes the overall metabolic equilibrium, affecting both oxygen reduction mechanisms in heterocysts as well as processes providing reducing equivalents for metabolic functions such as N2 fixation. We identify specific metabolic processes used by NHM5 to maintain a high rate of N2 fixation, and thereby potential targets for further improvement of nitrogenase based H2 photogeneration. These targets include, but are not limited to, components of the oxygen scavenging capacity and cell envelope of heterocysts and proteins directly or indirectly involved in reduced carbon transport from vegetative cells to heterocysts.