A point mutation in atpC1 raises the redox potential of the Arabidopsis chloroplast ATP synthase γ-subunit regulatory disulfide above the range of thioredoxin modulation

The light-dependent regulation of chloroplast ATP synthase activity depends on an intricate but ill defined interplay between the proton electrochemical potential across the thylakoid membrane and thioredoxin-mediated redox modulation of a cysteine bridge located on the ATP synthase γ-subunit. The abnormal light-dependent regulation of the chloroplast ATP synthase in the Arabidopsis thaliana cfq (coupling factor quick recovery) mutant was caused by a point mutation (G to A) in the atpC1 gene, which caused an amino acid substitution (E244K) in the vicinity of the redox modulation domain in the γ-subunit of ATP synthase. Equilibrium redox titration revealed that this mutation made the regulatory sulfhydryl group energetically much more difficult to reduce relative to the wild type (i.e. raised the E_m,7.9 by 39 mV). Enzymatic studies using isolated chloroplasts showed significantly lower light-induced ATPase and ATP synthase activity in the mutant compared with the wild type. The lower ATP synthesis capacity in turn restricted overall rates of leaf photosynthesis in the cfq mutant under low light. This work provides in situ validation of the concept that thioredoxin-dependent reduction of the γ-subunit regulatory disulfide modulates the proton electrochemical potential energy requirement for activation of the chloroplast ATP synthase and that the activation state of the ATP synthase can limit leaf level photosynthesis.