RuBisCO i.e. Ribulose-1, 5-Bisphosphate Carboxylase/Oxygenase (E.C. No. 4.1.1.39) is the enzyme involved in the catalysis of the initial step of Calvin cycle, the major carbon fixing pathway on earth. The photosynthetic productivity of several higher crops becomes limiting under ambient conditions because of an inherent defect in the design of RubisCO which possess equal affinity for molecular oxygen which promotes a wasteful oxygenation reaction instead of the desirable carboxylation. This problem is largely alleviated in lower organisms like algae, cyanobacteria and also several higher plants like the C4 plants because they have evolved mechanisms to concentrate carbon dioxide around RubisCO hence escaping its oxygenase activity. Since cyanobacteria are credited with being the first organisms to utilize water as electron donors and release oxygen in the atmosphere, they are expected to be the first to evolve RubisCO’s oxygenase activity and/or carbon concentration mechanism/s. In this context, analyzing a primitive cyanobacterium, Gloeobacter violaceus PCC 7421 could give great insights into the development of ccms and their evolutionary relations with RubisCO. An in silico analysis has revealed the C-termini of several rbcS (gene encoding small subunit of RubisCO) genes demonstrating homology with ccmM, a protein involved in the initial assembly of the carboxysomes, the CO2 concentrating complexes. The homology suggests an evolutionary link and perhaps a carbon concentrating role of the small subunit in accordance to the function played by ccmM in cyanobacteria. In addition to unravelling the above apparent linkage, we are characterizing the complete operons encoding the above complexes, i.e., the one encoding RubisCO and the one encoding CCM proteins. Data obtained using bioinformatic tools and preliminary results of the efforts to clone and characterize the above genes are presented.

Keywords: RubisCO; Carbon concentration mechanisms; carboxysomes, Gloeobacter violaceous