Characterization and physiological role of two types of chloroplastic fructose-1,6-bisphosphatases in Euglena gracilis

• We characterized two types of chloroplastic FBPase isoforms from Euglena gracilis.
• The EgFBPaseI might be partially regulated by redox modulation in vivo.
• The EgFBPaseII was more tolerant to oxidation than EgFBPaseI.
• The EgFBPaseI was critical for the Calvin cycle in Euglena chloroplasts.

The chloroplastic fructose-1,6-bisphosphatase (FBPase) is a late-limiting enzyme in the Calvin cycle. In the present study, we isolated and characterized the cDNAs encoding two types of chloroplastic FBPase isoforms (EgFBPaseI and II) from Euglena gracilis. The Km values of recombinant EgFBPaseI and EgFBPaseII for fructose 1,6-bisphosphate (Fru 1,6-P2) were 165 ± 17 and 2200 ± 200 μM, respectively. The activity of EgFBPaseI was inhibited by 1 mM H2O2 and recovered when incubated with DTT. The activity of EgFBPaseII was resistant to concentrations of H2O2 up to 1 mM, which was distinct from those of EgFBPaseI and spinach chloroplastic FBPase. The suppression of EgFBPaseI gene expression by gene silencing markedly decreased photosynthetic activity and inhibited cell growth. The results of the present study clearly demonstrated that EgFBPaseI played a critical role in photosynthesis in Euglena chloroplasts.