Release of the nocturnal inhibitor, car☐yarabinitol‐1 ‐phosphate, from ribulose bisphosphate car☐ylase/oxygenase by rubisco activase

Car☐yarabinitol‐1‐phosphate (CA1P) is a naturally occurring inhibitor of ribulose‐1,5‐bisphosphate car☐ylase/oxygenase (rubisco) which appears in the leaves of some plants during darkness and occupies the ribulose‐1,5‐bisphosphate binding site. The involvement of the rubisco activase protein in the regulation of rubisco activity by CA1P was investigated. Addition of ribulose bisphosphate (RuBP) or 6‐phosphogluconate partially reversed CA1P inhibition by displacing the inhibitor from rubisco. Rubisco activase also partially reversed the inhibition but rubisco activase and RuBP together almost completely reversed CA1P inhibition of rubisco. Rubisco activase did not metabolize CA1P. Inhibition of rubisco by car☐yarabinitol‐1,5‐bisphosphate was not reversed by rubisco activase in the presence or absence of RuBP. The data suggest that rubisco activase interacts with rubisco to alter the RuBP binding site, thereby increasing the dissociation rate of bound CA1P from the enzyme.