CbbQ is encoded by the gene located downstream of ribulose 1,5-bisphosphate carboxylase/oxygenase genes (cbbLS) in the thermophilic hydrogen-oxidizing bacterium, Hydrogenophilus thermoluteolus. The protein possesses two nucleotide-binding motifs in its amino acid sequence, and it posttranslationally activates RubisCO. We present ATP hydrolysis and binding of CbbQ. CbbQ releases Pi from ATP only in the presence of Mg2+. CbbQ interacts with an 2'(3')-O(2,4,6-trinitrophenyl)adenosine 5'-triphosphate in the presence or absence of Mg2+. The interaction with Mg2+ and/or a nucleotide induces a conformational change in CbbQ. Autophosphorylation of CbbQ occurs only in the absence of Mg2+. (C) 2002 Elsevier Science (USA).