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The D1 subunit of photosystem II (PSII) is subject to light-induced damage. In plants, D1 photodamage activates translation of chloroplastpsbAmRNA encoding D1, providing D1 for PSII repair. Three D1 assembly factors have been implicated in the regulatory mechanism: HCF244 and RBD1 activatepsbAtranslation, whereas HCF136 repressespsbAtranslation in the dark. To clarify the regulatory circuit, we analyzedpsbAribosome occupancy in dark-adapted and illuminatedrbd1andrbd1;hcf136double mutants in Arabidopsis and in Zm-hcf244and Zm-hcf244;Zm-hcf136double mutants in maize. The results show that RBD1 is required for light-inducedpsbAtranslation but has only a small effect onpsbAribosome occupancy in the dark. RBD1 is not required forpsbAtranslation when HCF136 is absent, indicating that RBD1 activatespsbAtranslation in the light by inhibiting HCF136’s repressive effect. By contrast, HCF244 is required to recruit ribosomes topsbAmRNA in light, dark, and in the absence of HCF136. We demonstrate further that HCF244 is not required for the translational activator HCF173 to bind thepsbA5’UTR. These results show that RBD1 is central to the perception of the D1 photodamage that triggers D1 synthesis and that it activatespsbAtranslation by relieving repression by an HCF136-dependent assembly intermediate. HCF244 activates downstream of those events without impacting HCF173’s binding topsbAmRNA. The results implicate a feature of nascent D1 that is affected by both HCF136 and RBD1 as the signal that reports D1 photodamage to regulatepsbAtranslation rate as needed for PSII repair.
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The psbA open reading frame acts in cis to toggle HCF173 from an activator to a repressor for light-regulated psbA translation in plants
Abstract The D1 subunit of photosystem II is subject to photooxidative damage. Photodamaged D1 must be replaced with nascent D1 to maintain photosynthesis. In plant chloroplasts, D1 photodamage regulates D1 synthesis by modulating translation initiation on psbA mRNA encoding D1, but the underlying mechanisms are unknown. Analyses of reporter constructs in transplastomic tobacco (Nicotiana tabacum) showed that the psbA translational regulator HCF173 activates via a cis-element in the psbA 5′-UTR. However, the psbA UTRs are not sufficient to program light-regulated translation. Instead, the psbA open reading frame represses translation initiation in cis, and D1 photodamage relieves this repression. HCF173 remains bound to the psbA 5′-UTR in the dark and truncation of HCF173 prevents repression in the dark, implicating HCF173 as a mediator of repression. We propose a model that accounts for these and prior observations, which is informed by structures of the Complex I assembly factor CIA30/NDUFAF1. We posit that D1 photodamage relieves a repressive cotranslational interaction between nascent D1 and HCF173's CIA30 domain, that the photosystem II assembly factor HCF136 promotes this repressive interaction, and that these events toggle HCF173 between activating and repressive conformations on psbA mRNA. These findings elucidate a translational rheostat that optimizes photosynthesis in response to shifting light conditions.
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- Award ID(s):
- 2034758
- PAR ID:
- 10582167
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- The Plant Cell
- Volume:
- 37
- Issue:
- 4
- ISSN:
- 1040-4651
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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