Characterization of a succinate dehydrogenase complex and respiratory electron transport in Synechocystis sp. PCC 6803

Respiration in cyanobacterial thylakoid membranes is interwoven with photosynthetic processes. The open reading frames sll1625 and sll0823, which have significant sequence similarity to genes coding for the FeS subunits of succinate dehydrogenase and fumarate reductase, subunits of complex II in mitochondria, were deleted singly in the cyanobacterium Synechocystis sp. PCC 6803. The deletions affected succinate and fumarate levels and the reduction kinetics of the plastoquinone pool. Rates of reduction of the plastoquinone pool by electrons from the respiratory chain were at least 4--5-fold slower in the double mutant than in wild type. These findings and organic acid levels following addition of citric acid cycle intermediates on either side of the succinate to fumarate reaction indicated that a succinate dehydrogenase activity had been blocked by the deletions and that 2-oxoglutarate is converted to succinate in vivo , even though a traditional 2-oxoglutarate dehydrogenase is lacking.; Deletion of any one respiratory complex caused an equivalent oxidation of the PQ pool, but the kinetics of PQ reduction differed. Analysis of organic acid levels and NAD and NADP redox state measurements indicated that succinate dehydrogenase is the main respiratory electron transfer pathway into the PQ pool, and that type I and type II NAD(P)H dehydrogenase regulate the reduction level of NADP and NAD, which in turn affects respiratory electron flow through succinate dehydrogenase because of substrate availability.; Succinate formation in the absence of a 2-oxoglutarate dehydrogenase, and the function of the putative protein Slr1022 possibly involved in this process were examined by organic and amino acid analysis of the succinate dehydrogenase-less and Slr1022-less strains. Our findings indicate that succinate in Synechocystis sp. PCC 6803 is formed from 2-oxoglutarate via the gamma-aminobutyrate shunt pathway and that the Slr1022 protein functions as a gamma-aminobutyrate transaminase, which catalyzes an intermediate step of this pathway.; Another open reading frame slr0201, was identified in the Synechocystis genome, sequence as the gene for a heterodisulfide reductase subunit. However, the corresponding protein was found to have similarity to a non-traditional succinate dehydrogenase subunit found mostly in Archaea. Slr0201 in this dissertation is shown to be part of the SDH complex and was necessary for succinate dehydrogenase, activity.