Closely Related Form I Ribulose Bisphosphate Carboxylase/Oxygenase Molecules That Possess Different CO2/O2Substrate Specificities

The deduced primary sequence (cbbLandcbbS) of form I ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) fromBradyrhizobium japonicumplaces this enzyme within the Type IC subgroup of red-like rubisco enzymes. In addition,B. japonicumappears to organize most of the structural genes of the Calvin–Benson–Bassham (CBB) pathway in at least one major operon. Functional expression and characterization of theB. japonicumandXanthobacter flavusenzymes from this group revealed that these molecules exhibit diverse kinetic properties despite their relatively high degree of sequence relatedness. Of prime importance was the fact that these closely related enzymes exhibited CO2and O2substrate specificities that varied from relatively low values [τ = (VcKo)/(VoKc) = 45] to values that approximated those obtained for higher plants (τ = 75). These results, combined with the metabolic and genetic versatility of the organisms from which these enzymes were derived, suggest a potential rich resource for future biological selection and structure–function studies aimed at elucidating structural features that govern key enzymological properties of rubisco.