Alteration of the regulation of Rubisco via Rubisco activase.

Abstract

Rubisco activase is regulated in vitro by the ADP/ATP ratio and reduction/oxidation of the larger isoform by thioredoxin-f, which alters the sensitivity of the Rubisco activation and ATPase activities of the protein to the ADP/ATP ratio. These two factors provide possible explanations for the observed response in plants of Rubisco activation to light intensity (redox) and for reduced activation when there are sink limitations and reduced triose phosphate utilization (ADP/ATP). Redox regulation of activase is eliminated if only the smaller isoform is present or critical cysteines, present only in the larger isoform, are replaced by alanine. Regulation by ADP/ATP ratio is altered by replacement of glutamine in a nucleotide binding loop (i.e. the P-loop) by either glutamate or aspartate. To directly address the relationships between the regulation of activase and the modulation of Rubisco activity in Arabidopsis, we transformed the rca-minus mutant, which does not express activase, to obtain plants that express various forms of activase. Light modulation of Rubisco activity was eliminated in plants without the larger isoform or if the cysteine residues were replaced by alanine. ADP/ATP regulation is more difficult to address, because plants expressing only the shorter form of activase with glutamine or aspartate substitution are also no longer light modulated. However, introduction of an unmodified larger isoform by genetically crossing these lines with plants expressing only the larger isoform restored light modulation. Thus, these plants may be suitable for future studies of the role of ADP/ATP regulation in the response to Rubisco to sink limitations.