In the cyanobacterial RuBisCO operon from Thermosynechococcus elongatus the rbcX gene is juxtaposed and cotranscribed with the rbcL and rbcS genes which encode large and small RuBisCO subunits, respectively. It has been suggested that the rbcX position is not random and that the RbcX protein could be a chaperone for RuBisCO. In this study, the RbcX protein from T. elongatus was overexpressed, purified and preliminary functional studies were conducted. The recombinant protein purified from Escherichia coli extracts was predominantly present in a soluble fraction in a dimeric form. Coexpression experiments have demonstrated that RbcX can mediate RbcL dimer (L(2)) formation, and that it is essential for the L(8) core complex assembly. This is the first characterization of the RbcX protein from a thermophilic organism.