Photosynthesis is a process used by plants and other organisms to convert the light energy captured from the sun into chemical energy that can be used to fuel the organism's activities. During this process, the obligation of the Calvin cycle is to fix carbon. In eukaryotes, the reactions of Calvin cycle are catalyzed by 11 enzymes, while, in prokaryotes, this number has decreased into 10. The cause of the smaller number is the functions of the two critical light regulated enzymes (FBPase and SBPase) in eukaryotes are performed by a dual-functional enzyme in prokaryotes (F/SBPase).
So, it is easy for people to assume that the FBPase and SBPase are diverged from a shared ancestor--the prokaryotes bifunctional F/SBPase of mitochondrial origin. Although this assumption has been accepted for a while, but some contradictions are keeping make it hard to stand, such as, why no specific affinity between SBPase and eubacterial FBPase or F/SBPase can be observed in the previous phylogenetic analyses.
To help settling down the argument, Dr. WEN Jianfan (Kunming Institute of Zoology, the CAS) and his research group recently have finished their phylogenetic analysis based on a much larger taxonomic sampling than previous work. “Out results are inspiring. They are quite helpful to picture a much more sophisticated Calvin cycle regulation in eukaryotes than we have even imagined”, said Dr. WEN.
By comparing the functional domains of prokaryotes’ F/SBPase from different organisms, their results for the first time indicated that prokaryotes bifunctional F/SBPase can be classified into two evolutionarily distant classes: Class I F/SBPase with typical FBPase domain and Class II F/SBPase with FBPase_glpX domain.
Whereas, eukaryotic FBPase and SBPase do not diverge from either of the two classes of F/SBPase but have two independent origins. SBPase share a common ancestor with the gluconeogenesis-specific Class I FBPase of epsilon-proteobacteria (or probably originated from that of the ancestor of epsilonproteobacteria), while FBPase arise from Class I FBPase of an unknown kind of eubacteria.Moreover, their findings also support that during the evolution of SBPase from eubacterial Class I FBPase, the SBP-dephosphorylation activity was acquired through the transition “from specialist to generalist”.
In summary, in this study, the strong evidence of the origins of two critical light-regulated enzymes in eukaryotes has been presented. And more importantly, it also indicated that the evolutionary substitution of the endosymbiotic-origin cyanobacterial bifunctional F/SBPase by the two light-regulated substrate-specific enzymes made the regulation of the Calvin cycle more delicate, which contributed to the evolution of eukaryotic photosynthesis and even the entire photosynthetic eukaryotes.
The main findings of this study have been published on BMC Evolutionary Biology (DOI: 10.1186/1471-2148-12-208) (http://www.biomedcentral.com/1471-2148/12/208/abstract).