Closely related form I ribulose bisphosphate carboxylase/oxygenase molecules that possess different CO2/O-2 substrate specificities

The deduced primary sequence (cbbL and cbbS) of form I ribulose 1,5-bisphos phate carboxylase/oxygenase (rubisco) from Bradyrhizobium japonicum places this enzyme within the Type IC subgroup of red-like rubisco enzymes. In add ition, B. japonicum appears to organize most of the structural genes of the Calvin-Benson-Bassham (CBB) pathway in at least one major operon, Function al expression and characterization of the B. japonicum and Xanthobacter fla vus enzymes from this group revealed that these molecules exhibit diverse k inetic properties despite their relatively high degree of sequence relatedn ess. Of prime importance was the fact that these closely related enzymes ex hibited CO2 and O-2 substrate specificities that varied from relatively low values [tau = (VcKo)/(VoKc) = 45] to values that approximated those obtain ed for higher plants (tau = 75). These results, combined with the metabolic and genetic versatility of the organisms from which these enzymes were der ived, suggest a potential rich resource for future biological selection and structure-function studies aimed at elucidating structural features that g overn key enzymological properties of rubisco. (C) 1999 Academic Press.