Increased activity of the non-regulated enzymes fructose-1,6-bisphosphate aldolase and triosephosphate isomerase in Anabaena sp. strain PCC 7120 increases photosynthetic yield

Non-regulated enzymes in the Calvin cycle are generally presumed to be less important for the regulation of photosynthetic yield. Here, to investigate the relationship between the activity of non-regulated enzymes and photosynthetic yield, two non-regulated enzymes in the Calvin cycle—a rice cytosolic fructose-1,6-bisphosphate aldolase (FBA) and a spinach chloroplast triosephosphate isomerase (TPI)—were cloned and co-expressed in cells of the cyanobacterium Anabaena sp. strain PCC 7120. The activity of FBA and TPI and the photosynthetic yield reflected by photosynthetic O₂ evolution and cell dry weight were measured and compared between wild-type and transgenic cells. Our results demonstrated that the activity of FBA and TPI were increased in transgenic cells relative to wild-type cells, and that activity was further increased in a transgenic strain harboring two sets of FBA-TPI tandem genes relative to cells containing one copy of the FBA-TPI tandem gene. The increased activity of FBA and TPI in Anabaena sp. strain PCC 7120 increased photosynthetic yield, with increased activity levels correlating closely with the degree of changes in photosynthetic yield. This implies that the photosynthetic yield is limited by the activity of the non-regulated enzymes FBA and TPI, and that the endogenous activity of non-regulated enzymes is not sufficient to increase photosynthetic yield. We discuss the various roles of FBA and TPI, and regulated and non-regulated enzymes, in modulating photosynthetic yield. W. Ma and L. Wei contributed equally to this work.