Fine control of citrate synthase activity in blue-green algae

Summary The citrate synthases of four blue-green algae, two unicellular (Aphanocapsa spp.) and two filamentous (Nostoc sp., Phormidium sp.) were inhibited by -ketoglutarate but not by NADH. This control of citrate synthase activity reflects the lack of -ketoglutarate dehydrogenase in blue-green algae and the strictly biosynthetic role played by the glutamate branch of the tricarboxylic acid cycle. The citrate synthases were also inhibited by ATP and the enzyme of one of the unicellular organisms was also sensitive to inhibition by NADPH. These effectors may function in regulating the flow of fixed carbon into lipids rather than the glutamate family of amino acids.Contribution No. 1649 from the University of Miami, Rosenstiel School of Marine and Atmospheric Science, 10 Rickenbacker Causeway, Miami, Florida 33149, U.S.A.