It is generally accepted that cyanobacteria have an incomplete tricarboxylic acid (TCA) cycle because they lack 2-oxoglutarate dehydrogenase and thus cannot convert 2-oxoglutarate to succinyl-coenzyme A (CoA). Genes encoding a novel 2-oxoglutarate decarboxylase and succinic semialdehyde dehydrogenase were identified in the cyanobacterium Synechococcus sp. PCC 7002. Together, these two enzymes convert 2-oxoglutarate to succinate and thus functionally replace 2-oxoglutarate dehydrogenase and succinyl-CoA synthetase. These genes are present in all cyanobacterial genomes except those of Prochlorococcus and marine Synechococcus species. Closely related genes occur in the genomes of some methanogens and other anaerobic bacteria, which are also thought to have incomplete TCA cycles.
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