期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2016
卷号：113
期号：49
页码：14019-14024
DOI：10.1073/pnas.1610758113
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceEukaryotic phytoplankton of the red plastid lineage dominate the oceans and are responsible for a significant proportion of global photosynthetic CO2 fixation. In contrast to their ecological importance, relatively little is known about the biochemical properties of their carbon dioxide fixation machinery. In plants, the carbon dioxide fixing enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) forms inactive complexes with sugar phosphates and needs to be constantly remodeled by the motor protein rubisco activase (Rca). Here we show that in red algae, rubisco also forms inhibited complexes, which can be rescued by a convergently evolved Rca similar to one described in photosynthetic bacteria. Some red algal rubiscos are a target for crop improvement strategies, because their kinetics are better suited to the current atmospheric gas composition. The photosynthetic CO2-fixing enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) is inhibited by nonproductive binding of its substrate ribulose-1,5-bisphosphate (RuBP) and other sugar phosphates. Reactivation requires ATP-hydrolysis-powered remodeling of the inhibited complexes by diverse molecular chaperones known as rubisco activases (Rcas). Eukaryotic phytoplankton of the red plastid lineage contain so-called red-type rubiscos, some of which have been shown to possess superior kinetic properties to green-type rubiscos found in higher plants. These organisms are known to encode multiple homologs of CbbX, the -proteobacterial red-type activase. Here we show that the gene products of two cbbX genes encoded by the nuclear and plastid genomes of the red algae Cyanidioschyzon merolae are nonfunctional in isolation, but together form a thermostable heterooligomeric Rca that can use both -proteobacterial and red algal-inhibited rubisco complexes as a substrate. The mechanism of rubisco activation appears conserved between the bacterial and the algal systems and involves threading of the rubisco large subunit C terminus. Whereas binding of the allosteric regulator RuBP induces oligomeric transitions to the bacterial activase, it merely enhances the kinetics of ATP hydrolysis in the algal enzyme. Mutational analysis of nuclear and plastid isoforms demonstrates strong coordination between the subunits and implicates the nuclear-encoded subunit as being functionally dominant. The plastid-encoded subunit may be catalytically inert. Efforts to enhance crop photosynthesis by transplanting red algal rubiscos with enhanced kinetics will need to take into account the requirement for a compatible Rca.
