In vitro activity of reconstituted rubisco enzyme from Gloeobacter violaceus

RuBisCO (Ribulose 1,5 bisphosphate carboxylase/oxygenase) by virtue of its dual specificity towards oxygen and carbon dioxide is an important rate-limiting step in photosynthesis and is believed to be the key factor for limited productivity of higher plants and algae. The photoautotrophic growth rate of cyanobacteria is a culmination of several factors including, rates of photosynthetic reactions, stress combating mechanisms and basic biomass generation metabolism in combination with optimal nutrient availability, irradiance, gaseous environment, etc. In case of cyanobacteria, the effect of RuBisCO in affecting the multiplication rate has been observed to show varied response. The current paper presents the RuBisCO activity of an early diverging cyanobacterium, Gloeobacter violaceus PCC 7421 and also compares the growth rates and RuBisCO activity of various cyanobacteria. A spectrophotometric estimation in a coupled enzyme assay system of the heterologous expressed G. violaceus PCC 7421 RuBisCO in E. coli, upon purification, revealed a carboxylation activity of LSu to be 5 nMol of phosphoglycerate min^-1 mg^-1 of protein, which is in coherence with the organism's slow growth rate. Further, the in vitro complementation of RbcL with RbcS in presence of RbcX of G. violaceus facilitated partial reconstitution of the protein and was hence found to cause a four-fold enhancement in its specific activity. The unique characteristics of the primitive cyanobacteria, such as, absence of thylakoids, lack of several photosystem constituting genes, slow carboxylation rate, pose limitation for its rapid multiplication. The RuBisCO carboxylation rate is observed as not the sole but an important parameter for obtaining optimal cell multiplication rates in photo-autotrophically multiplying cyanobacteria.