title: Non-growing Rhodopseudomonas palustris increases the hydrogen gas yield from acetate by shifting from the glyoxylate shunt to the tricarboxylic acid cycle.
abstract: When starved for nitrogen, non-growing cells of the photosynthetic bacterium Rhodopseudomonas palustris continue to metabolize acetate and produce H2, an important industrial chemical and potential biofuel. The enzyme nitrogenase catalyzes H2 formation. The highest H2 yields are obtained when cells are deprived of N2 and thus use available electrons to synthesize H2 as the exclusive product of nitrogenase. To understand how R. palustris responds metabolically to increase H2 yields when it is starved for N2, and thus not growing, we tracked changes in biomass composition and global transcript levels. In addition to a 3.5-fold higher H2 yield by non-growing cells we also observed an accumulation of polyhydroxybutyrate to over 30% of the dry cell weight. The transcriptome of R. palustris showed down-regulation of biosynthetic processes and up-regulation of nitrogen scavenging mechanisms in response to N2 starvation but gene expression changes did not point to metabolic activities that could generate the reductant necessary to explain the high H2 yield. We therefore tracked (13)C-labeled acetate through central metabolic pathways. We found that non-growing cells shifted their metabolism to use the tricarboxylic acid cycle to metabolize acetate in contrast to growing cells, which used the glyoxylate cycle exclusively. This shift enabled cells to more fully oxidize acetate, providing the necessary reducing power to explain the high H2 yield.
authors.affiliation: From the Department of Biology, Indiana University, Bloomington, Indiana 47405.
authors.forename: James B
authors.initials: JB
authors.lastname: McKinlay
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Oda
authors.forename: Martin
authors.initials: M
authors.lastname: Rühl
authors.forename: Amanda L
authors.initials: AL
authors.lastname: Posto
authors.forename: Uwe
authors.initials: U
authors.lastname: Sauer
authors.forename: Caroline S
authors.initials: CS
authors.lastname: Harwood
mesh_terms: D000085:Acetates / Q000378:metabolism*; D002952:Citric Acid Cycle / Q000502:physiology*; D006038:Glyoxylates / Q000378:metabolism*; D006859:Hydrogen / Q000378:metabolism*; D006885:Hydroxybutyrates / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism; D010084:Oxidation-Reduction; D011091:Polyesters / Q000378:metabolism; D012241:Rhodopseudomonas / Q000378:metabolism*; D059467:Transcriptome / Q000502:physiology; D000094122:Polyhydroxybutyrates
chemical_list: D000085:Acetates; D006038:Glyoxylates; D006859:Hydrogen; D006885:Hydroxybutyrates; D009584:Nitrogen; D011091:Polyesters; C003182:poly-beta-hydroxybutyrate; D000094122:Polyhydroxybutyrates; C031150:glyoxylic acid
keywords: Bacterial Metabolism; Bacterial Starvation; Biofuel; Hydrogen Gas; Metabolic Engineering; Metabolic Flux Analysis; Metabolic Regulation; Metabolic Tracers; Nitrogenase; Transcriptomics