| Summary: | The key photosynthetic CO2 fixing enzyme Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) is prone to be inhibited by its own substrate RuBP and other sugar phosphates. In diverse organisms, various molecular chaperones termed the Rubisco activases have been recruited to remodel inhibited Rubisco complexes leading to the release of the inhibitors. Although red-type Rubiscos are a potential target in plant biotechnology to enhancing photosynthetic efficiency, their regulation is poorly studied. Here we biochemically characterized the two red-type Rubisco activase isoforms encoded by the thermophilic red algae Cyanidioschyzon merolae, as well as a homologue found in α-cyanobacteria. The individual algal isoforms are inactive in isolation, but when mixed form a functional hetero-oligomeric activase. Mutagenesis of key functional motifs indicates that ATP hydrolysis of the subunits is highly coordinated and that the nuclear-genome encoded isoform is more critical for activase function than the plastid-genome encoded counterpart. Our findings also suggest broad substrate compatibility among red-type Rubisco activases.
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