Succinate : quinol oxidoreductases in the cyanobacterium Synechocystis sp strain PCC 6803: Presence and function in metabolism and electron transport

The open reading frames sll1625 and sll0823, which have significant sequenc e similarity to genes coding for the FeS subunits of succinate dehydrogenas e and fumarate reductase, were deleted singly and in combination in the cya nobacterium Synechocystis sp, strain PCC 6803. When the organic acid conten t in the Delta sll1625 and Delta sll0823 strains was analyzed, a 100-fold d ecrease in succinate and fumarate concentrations was observed relative to t he wild type. A similar analysis for the Delta sll1625 Delta sll0823 strain revealed that 17% of the wild-type succinate levels remained, while only 1 to 2% of the wild-type fumarate levels were present. Addition of 2-oxoglut arate to the growth media of the double mutant strain prior to analysis of organic acids in cells caused succinate to accumulate. This indicates that succinate dehydrogenase activity had been blocked by the deletions and that 2-oxoglutarate can be converted to succinate in vivo in this organism, eve n though a traditional 2-oxoglutarate dehydrogenase is lacking, In addition , reduction of the thylakoid plastoquinone pool in darkness in the presence of KCN was up to fivefold slower in the mutants than in the wild type. Mor eover, in vitro succinate dehydrogenase activity observed in wild-type memb ranes is absent from those isolated from the double mutant and reduced in t hose from the single mutants, further indicating that the sll1625 and sll08 23 open reading frames encode subunits of succinate dehydrogenase complexes that are active in the thylakoid membrane of the cyanobacterium.