Subsaturating RibuIose-l,5=Bisphosphate Concentration Promotes lnactivation of Ribulose-l,5-Bisphosphate Carboxylase/Oxygenase (Rubisco)'

We developed a continuous-addition method for maintaining subsaturating concentrations of ribulose-1,5-bisphosphate (RuBP) for severa1 minutes, while simultaneously monitoring i t s consumption by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). This method enabled us to observe the effects of subsaturating RuBP and CO, concentrations on the activity of Rubisco during much longer periods than previously studied. At saturating CO,, the activity of the,enzyme declined faster when RuBP was maintained at concentrations near i t s K,,, value than when RuBP was saturating. At saturating RuBP, activity declined faster at limiting than at saturating CO,, i n accordance with previous observations. The most rapid decline i n activity occurred when both CO, and RuBP concentrations were subsaturating. The activity loss was accompanied by decarbamylation of the enzyme, even though the enzyme was maintained at the same CO, concentration before and after exposure to RuBP. Rubisco activase ameliorated the decline in activity at subsaturating CO, and RuBP concentrations. The results are consistent with a proposed mechanism for regulating the carbamylation of Rubisco, which postulates that Rubisco activase counteracts Rubisco's unfavorable carbamylation equilibrium in the presence of RuBP by accelerating, in an ATP-dependent manner, the release of RuBP from i t s complex with uncarbamylated sites.