Abstract
Photosystem II (PSII) in chloroplasts and cyanobacteria contains approximately fifteen core proteins, which organize numerous pigments and prosthetic groups that mediate the light-driven water-splitting activity that drives oxygenic photosynthesis. The PSII reaction center protein D1 is subject to photodamage, whose repair requires degradation of damaged D1 and its replacement with nascent D1. Mechanisms that couple D1 synthesis with PSII assembly and repair are poorly understood. We address this question by using ribosome profiling to analyze the translation of chloroplast mRNAs in maize and Arabidopsis mutants with defects in PSII assembly. We found that OHP1, OHP2, and HCF244, which comprise a recently elucidated complex involved in PSII assembly and repair, are each required for the recruitment of ribosomes to psbA mRNA, which encodes D1. By contrast, HCF136, which acts upstream of the OHP1/OHP2/HCF244 complex during PSII assembly, does not have this effect. The fact that the OHP1/OHP2/HCF244 complex brings D1 into proximity with three proteins with dual roles in PSII assembly and psbA ribosome recruitment suggests that this complex is the hub of a translational autoregulatory mechanism that coordinates D1 synthesis with need for nascent D1 during PSII biogenesis and repair.
Highlights
Photosystem II (PSII) is a large protein–pigment complex whose light-driven water splitting activity is the foundation of oxygenic photosynthesis
Our results show that maize HCF244 is required for psbA translation initiation, consistent with the prior conclusions from polysome data in Arabidopsis [22]
In contrast with previous conclusions [14], our results indicate that OHP1 and OHP2 are likewise required for the recruitment of ribosomes to the psbA mRNA
Summary
Photosystem II (PSII) is a large protein–pigment complex whose light-driven water splitting activity is the foundation of oxygenic photosynthesis (reviewed in [1,2]). PSII localizes to thylakoid membranes of cyanobacteria and chloroplasts, where it consists of a highly conserved core complex surrounded by distinct light harvesting complexes. The PSII core contains approximately fifteen proteins in 1:1 stoichiometry, which serve as scaffolds to organize the numerous pigments and prosthetic groups that mediate light-absorption, charge separation and electron transport. This structure is assembled in an ordered pathway with the assistance of accessory factors that are, in many cases, conserved between cyanobacteria and chloroplasts [3]. PSII is a highly dynamic structure due to the damaging effects of light on its D1 reaction center protein. PSII repair involves an elaborate process that includes partial disassembly of the complex, proteolysis of damaged D1, new D1 synthesis, and re-assembly of the complex [4,5,6]
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