Abstract Rubisco activase is an ATP-dependent chaperone that facilitates dissociation of inhibitory sugar phosphates from the catalytic sites of ribulose-1,5-bisphosphate carboxylase/oxygenase during photosynthesis. In Arabidopsis, Rubisco activase is negatively regulated by dark-dependent phosphorylation of threonine 78. The prevalence of threonine 78 in Rubisco activase was investigated across sequences from 91 plant species, finding 29 (~32%) species shared a threonine in the same position.