| 其他摘要 | Calvin cycle is the only carbon dioxide fixation pathway in eukaryotes. The process of Calvin cycle had been illustrated by Calvin M in 1956. The origins of most Calvin cycle enzymes are clear, but how did FBPase (Fructose-1, 6-bisphosphatase) and SBPase (sedoheptulose-1,7-bisphosphatase) originate remains an open question. In the present work, firstly, we characterized FBPase II in the model algae of Chlamydomonas reinhardtii and Volvox carteri for the first time. Then, by phylogenetic analysis, we tried to provide some new insights into the origin of eukaryal FBPase and SBPase.
FBPase is an important regulatory enzyme. In photosynthetic eukaryotes, it generally has two isoforms, chloroplastic and cytosolic FBPase, involved in Calvin cycle and gluconeogenesis, respectively. While, in the green algae Chlamydomonas reinhardtii and Volvox carteri, only the chloroplastic FBPase gene can be found in their genomes. Surprisingly, by surveying their genome databases, we found they have another novel FBPase gene, which is homologous to glpX-like gene that was only found in prokaryotes previously. No other eukaryotes with genome databases were found having such a prokaryotic FBPase gene in our investigation. EST data and our RT-PCR assay showed this gene is actively transcribed in two chlorophytes. Sequence analysis indicated the enzyme encoded by it contains both FBPase_glpX domain and Li+-sensitive phosphatase motif, suggesting the enzyme is a class II FBPase, and we called it FBPase2. Possessing an N-terminal presequence, the two enzymes were predicted to target to chloroplast by three target prediction programs.
Furthermore, our phylogenetic analysis indicated FBPase2 have the closest relationship with FBPase II of actinobacteridae. Together with the evidence that they share a unique motif, it is suggested FBPase2 might arise through an ancient lateral gene transfer (LGT) from the ancestor of actinobacteridae to the common ancestor of C. reinhardtii and V. carteri. Due to actinobacteridae FBPase II specifically share the characteristics with plant cytosolic FBPase, such as being inhibited by AMP and
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fructose-2,6-bisphoahate, we propose that the novel class II FBPase may act as a functional substitute of cytosolic FBPase to engage in gluconeogenesis in the chloroplasts rather than in the cytosol of the two chlorophytes. Therefore, our observations can not only explain why C. reinhardtii be able to synthesize sucrose without cytosolic FBPase, but also interpret why the organism can synthesize glucose with acetate as its sole carbon source under nonphotosynthetical growth in the dark.
The dephosphorylation of fructose-1,6-bisphosphate and sedoheptulose-1, 7-bisphosphate are two important steps in the Calvin cycle. In eubacteria, they are catalyzed by a common bifunctional F/SBPase; while in photosynthetic eukaryotes, by two substrate-specific enzymes, chloroplastic FBPase and SBPase, respectively. It has been indicated previously that FBPase and SBPase share a common evolutionary origin and acquired their substrate specificities after divergence from a common bifunctional F/SBPase ancestor. However, in the present work, firstly, our domain analysis showed that eubacterial F/SBPase can also be divided into two evolutionarily distant classes (Class I and Class II); then, by using more representative sequences from diverse lineages of eubacteria than in previous studies, our phylogeny analysis indicated that eukaryal FBPase and SBPase did not diverge from a common bifunctional F/SBPase ancestor (neither Class I nor Class II), but have two independent eubacterial origins: FBPase have an unkown bacterial FBPase origin, while SBPase might arise from epsilon-proteobacteria FBPase through an unknown mechanism. The origins of SBPase activity in eubacteria and eukaryotes are also analyzed. Finally, through analyzing the origin of eukaryal FBPase and SBPase, the origin of Calvin cycle in photosynthetic eukaryotes was discussed. |
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