Abstract
It has been believed that the Calvin cycle composed of eleven enzymes was initially completed in cyanobacteria. In particular, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) as a CO2-fixing enzyme and phosphoribulokinase (PRK) catalyzing the ribulose-1,5-bisphosphate (RuBP) production are enzymes unique to this cycle. The primitive cyanobacterium Gloeobactor violaceus possesses prk homologue genes, prk2 and 3 in addition of prk1 for a typical cyanobacterial photosynthetic PRK. The Vmax and Km(Ru5P) values of PRK1 were 200 μmol/min/mg protein and 0.28 mM, respectively, similar to those of typical cyanobacterial PRK. PRK2 did not show activity at all. Interestingly, PRK3 showed significant PRK activity, and the Vmax and Km(Ru5P 0.5) values were 23.5 μmol/min/mg protein and 5 mM, respectively. In phylogenetic tree, G. violaceus PRK3 and PRK homologues from other organisms form a clade distinct from plant and bacterial lineages, suggesting that G. violaceus PRK3 is a novel PRK form. In order to study on a relationship between the novel type of PRK and molecular evolution of the Calvin cycle, we are analyzing G. violaceus PRK3 homologues.
