Disruption of Hydrogen Gas Synthesis Enhances the Cellular Levels of NAD(P)H, Glycogen, Poly(3-hydroxybutyrate) and Photosynthetic Pigments Under Specific Nutrient Condition(s) in Cyanobacterium Synechocystis sp. PCC 6803

Abstract

In photoautotrophic Synechocystis sp. PCC 6803, NADPH is generated from photosynthesis and utilized in various metabolism, including the biosynthesis of glyceraldehyde 3-phosphate (the upstream substrate for carbon metabolism), poly(3-hydroxybutyrate) (PHB), photosynthetic pigments, and hydrogen gas (H2). Redirecting NADPH flow from one biosynthesis pathway to another has yet to be studied. Synechocystis's H2 synthesis, one of the pathways consuming NAD(P)H, was disrupted by the inactivation of hoxY and hoxH genes encoding the two catalytic subunits of hydrogenase. Such inactivation with a complete disruption of H2 synthesis led to 1.4-, 1.9-, and 2.1-fold increased cellular NAD(P)H levels when cells were cultured in normal medium (BG11), the medium without nitrate (-N), and the medium without phosphate (-P), respectively. After 49-52 d of cultivation in BG11 (when the nitrogen source in the media was depleted), the cells with disrupted H2 synthesis had 1.3-fold increased glycogen level compared to wild type of 83-85% (w/w dry weight), the highest level reported for cyanobacterial glycogen. The increased glycogen content observed by transmission electron microscopy was correlated with the increased levels of glucose 6-phosphate and glucose 1-phosphate, the two substrates in glycogen synthesis. Disrupted H2 synthesis also enhanced PHB accumulation up to 1.4-fold under -P and 1.6-fold under -N and increased levels of photosynthetic pigments (chlorophyll a, phycocyanin, and allophycocyanin) by 1.3- to 1.5-fold under BG11. Thus, disrupted H2 synthesis increased levels of NAD(P)H, which may be utilized for the biosynthesis of glycogen, PHB, and pigments. This strategy might be applicable for enhancing other biosynthetic pathways that utilize NAD(P)H.