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Title: Light-induced psbA translation in plants is triggered by photosystem II damage via an assembly-linked autoregulatory circuit

The D1 reaction center protein of photosystem II (PSII) is subject to light-induced damage. Degradation of damaged D1 and its replacement by nascent D1 are at the heart of a PSII repair cycle, without which photosynthesis is inhibited. In mature plant chloroplasts, light stimulates the recruitment of ribosomes specifically topsbAmRNA to provide nascent D1 for PSII repair and also triggers a global increase in translation elongation rate. The light-induced signals that initiate these responses are unclear. We present action spectrum and genetic data indicating that the light-induced recruitment of ribosomes topsbAmRNA is triggered by D1 photodamage, whereas the global stimulation of translation elongation is triggered by photosynthetic electron transport. Furthermore, mutants lacking HCF136, which mediates an early step in D1 assembly, exhibit constitutively highpsbAribosome occupancy in the dark and differ in this way from mutants lacking PSII for other reasons. These results, together with the recent elucidation of a thylakoid membrane complex that functions in PSII assembly, PSII repair, andpsbAtranslation, suggest an autoregulatory mechanism in which the light-induced degradation of D1 relieves repressive interactions between D1 and translational activators in the complex. We suggest that the presence of D1 in this complex coordinates D1 synthesis with the need for nascent D1 during both PSII biogenesis and PSII repair in plant chloroplasts.

 
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NSF-PAR ID:
10184880
Author(s) / Creator(s):
;
Publisher / Repository:
Proceedings of the National Academy of Sciences
Date Published:
Journal Name:
Proceedings of the National Academy of Sciences
Volume:
117
Issue:
35
ISSN:
0027-8424
Page Range / eLocation ID:
p. 21775-21784
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Summary

    Synthesis of the D1 reaction center protein of Photosystem II is dynamically regulated in response to environmental and developmental cues. In chloroplasts, much of this regulation occurs at the post‐transcriptional level, but the proteins responsible are largely unknown. To discover proteins that impactpsbAexpression, we identified proteins that associate with maizepsbAmRNA by: (i) formaldehyde cross‐linking of leaf tissue followed by antisense oligonucleotide affinity capture ofpsbAmRNA; and (ii) co‐immunoprecipitation with HCF173, apsbAtranslational activator that is known to bindpsbAmRNA. The S1 domain protein SRRP1 and two RNA Recognition Motif (RRM) domain proteins, CP33C and CP33B, were enriched with both approaches. Orthologous proteins were also among the enriched protein set in a previous study in Arabidopsis that employed a designer RNA‐binding protein as apsbARNA affinity tag. We show here that CP33B is bound topsbAmRNAin vivo,as was shown previously for CP33C and SRRP1. Immunoblot, pulse labeling, and ribosome profiling analyses of mutants lacking CP33B and/or CP33C detected some decreases in D1 protein levels under some conditions, but no change inpsbARNA abundance or translation. However, analogous experiments showed that SRRP1 repressespsbAribosome association in the dark, repressesycf1ribosome association, and promotes accumulation ofndhCmRNA. As SRRP1 is known to harbor RNA chaperone activity, we postulate that SRRP1 mediates these effects by modulating RNA structures. The uncharacterized proteins that emerged from our analyses provide a resource for the discovery of proteins that impact the expression ofpsbAand other chloroplast genes.

     
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  2. Summary

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