Succinate dehydrogenase and other respiratory pathways in thylakoid membranes of Synechocystis sp strain PCC 6803: Capacity comparisons and physiological function

Respiration in cyanobacterial thylakoid membranes is interwoven with photos ynthetic processes. We have constructed a range of mutants that are impaire d in several combinations of respiratory and photosynthetic electron transp ort complexes and have examined the relative effects on the redox state of the plastoquinone (PQ) pool by using a quinone electrode. Succinate dehydro genase has a major effect on the PQ redox poise, as mutants lacking this en zyme showed a much more oxidized PQ pool. Mutants lacking type I and II NAD (P)H dehydrogenases also had more oxidized PQ pools. However, in the mutant lacking type I NADPH dehydrogenase, succinate was essentially absent and e ffective respiratory electron donation to the PQ pool could be established after addition of 1 mM succinate. Therefore, lack of the type I NADPH dehyd rogenase had an indirect effect on the PQ pool redox state. The electron do nation capacity of succinate dehydrogenase was found to be an order of magn itude larger than that of type I and II NAD(P)H dehydrogenases. The reason for the oxidized PQ pool upon inactivation of type II NADH dehydrogenase ma y be related to the facts that the NAD pool in the cell is much smaller tha n that of NADP and that the NAD pool is fully reduced in the mutant without type II NADH dehydrogenase, thus causing regulatory inhibition. The result s indicate that succinate dehydrogenase is the main respiratory electron tr ansfer pathway into the PQ pool and that type I and II NAD(P)H dehydrogenas es regulate the reduction level of NADP and NAD, which, in turn, affects re spiratory electron flow through succinate dehydrogenase.