Lack of Light/Dark Regulation of Enzymes Involved in the Photosynthetic Carbon Reduction Cycle in Cyanobacteria, Synechococcus PCC 7942 and Synechocystis PCC 6803.

During the photosynthetic carbon reduction (PCR) cycle of Synechococcus PCC 7942 and Synechocystis PCC 6803, fructose-1,6-bisphosphatase, NADP(+)-glyceraldehyde-3-phosphate dehydrogenase, and ribulose-5-phosphate kinase were not sensitive to treatment with dithiothreitol (DTT), a reducing agent, in vitro and were not regulated by light in vivo, unlike the chloroplastic enzymes of higher plants. These results indicate that the PCR cycle in the cyanobacterial cells may not be actually regulated by light in vivo even if the ferredoxin/thioredoxin system is present.