Increased activity of the tandem fructose-1,6-bisphosphate aldolase,triosephosphate isomerase and fructose-1,6-bisphosphatase enzymes in <Emphasis Type="Italic">Anabaena</Emphasis> sp. strain PCC 7120 stimulates photosynthetic yield

The regulation of photosynthetic yield at the genetic level has largely focused on manipulation of the catalytic enzymes in the Calvin cycle by genetic engineering. In order to investigate the contribution of increased enzymatic activity in the Calvin cycle on photosynthetic yield, the rice fructose-1,6-bisphosphate aldolase (FBA), spinach triosephosphate isomerase (TPI) and wheat fructose-1,6-bisphosphatase (FBPase) genes were cloned in tandem and co-overexpressed in cyanobacterium Anabaena sp. strain PCC 7120 cells. The enzymatic activities of FBA, TPI and FBPase, as well as sedoheptulose-1,7-bisphosphatase (SBPase), were remarkably increased in transgenic cells relative to the wild-type. The photosynthetic yield, as reflected by photosynthetic O₂ evolution and dry cellular weight, was also markedly increased in transgenic cells versus wide-type cells. The activity of SBPase is considered the most important factor for ribulose-1,5-bisphosphate (RuBP) regeneration in the Calvin cycle, and increased activity of TPI alone in transgenic cells does not stimulate photosynthetic yield. Thus, the increased activity of FBA and FBPase, but not TPI, significantly improved photosynthetic yield in transgenic cells by stimulating SBPase activity and consequently accelerating the RuBP regeneration rate.