Fructose-1,6-bisphosphatase (FBPase) fine-tunes heterotrophic growth in cyanobacteria

Cyanobacteria switch their carbon metabolism between photoautotrophy and heterotrophy during diurnal cycles. In cyanobacteria, the classical glycolytic control point is characterized by two catabolic phosphofructokinases (PFKs) and a bifunctional anabolic fructose-1,6-biphosphatase/sedoheptulose-1,7-biphosphatase (F/SBPase; slr2094) catalyzing two key reactions in the Calvin-Benson-Bassham (CBB) cycle. In addition, Synechocystis possesses a fructose-1,6-bisphosphatase (FBPase; slr0952) with yet unknown physiological function and biochemical properties. Hence, our aim was to investigate the FBPase and the interplay of the four enzymes in photoautotrophic and heterotrophic carbon metabolism. We discovered that FBPase is specific for FBP, showing no SBPase activity, and unlike F/SBPase does not exhibit any biochemical regulatory properties. Growth studies with deletion mutants revealed that FBPase and PFKs play a major role under heterotrophic conditions. In contrast to F/SBPase, FBPase is not involved in the CBB cycle, but instead fine-tunes heterotrophic growth. Transaldolase cannot replace the function of SBPase in the CBB cycle. In conclusion, the classical Embden-Meyerhoff-Parnass pathway control point, which is known to be mediated by the antagonistic enzyme pair PFK and FBPase in heterotrophic bacteria and eukaryotes, is also present in Synechocystis. We found redox-insensitive FBPases from plant chloroplasts to be closely related to Synechocystis FBPase, indicating that they might serve a similar function. HighlightSynechocystis fructose-1,6-bisphosphatase (slr0952) is unlike fructose-1,6-biphosphatase/sedoheptulose-1,7-biphosphatase (slr2094) monofunctional, not redox-regulated, and supports heterotrophy in darkness, despite catalyzing an anabolic reaction. Thus, presumably Slr2094 alone drives two Calvin-Benson-Bassham cycle key reactions.