Rojas Margarita, Williams-Carrier Rosalind, Chotewutmontri Prakitchai, Belcher Susan, Boyce Emily, Barkan Alice
Institute of Molecular Biology, University of Oregon, Eugene, OR 97403.
Proc Natl Acad Sci U S A. 2025 Aug 26;122(34):e2423694122. doi: 10.1073/pnas.2423694122. Epub 2025 Aug 19.
The D1 subunit of photosystem II (PSII) is subject to light-induced damage. In plants, D1 photodamage activates translation of chloroplast mRNA encoding D1, providing D1 for PSII repair. Three D1 assembly factors have been implicated in the regulatory mechanism: HCF244 and RBD1 activate translation, whereas HCF136 represses translation in the dark. To clarify the regulatory circuit, we analyzed ribosome occupancy in dark-adapted and illuminated and double mutants in Arabidopsis and in Zm- and Zm-Zm- double mutants in maize. The results show that RBD1 is required for light-induced translation but has only a small effect on ribosome occupancy in the dark. RBD1 is not required for translation when HCF136 is absent, indicating that RBD1 activates translation in the light by inhibiting HCF136's repressive effect. By contrast, HCF244 is required to recruit ribosomes to mRNA in light, dark, and in the absence of HCF136. We demonstrate further that HCF244 is not required for the translational activator HCF173 to bind the 5'UTR. These results show that RBD1 is central to the perception of the D1 photodamage that triggers D1 synthesis and that it activates translation by relieving repression by an HCF136-dependent assembly intermediate. HCF244 activates downstream of those events without impacting HCF173's binding to mRNA. The results implicate a feature of nascent D1 that is affected by both HCF136 and RBD1 as the signal that reports D1 photodamage to regulate translation rate as needed for PSII repair.
光系统II（PSII）的D1亚基会受到光诱导损伤。在植物中，D1光损伤会激活叶绿体中编码D1的mRNA的翻译，为PSII修复提供D1。三种D1组装因子参与了这一调控机制：HCF244和RBD1激活翻译，而HCF136在黑暗中抑制翻译。为阐明这一调控回路，我们分析了拟南芥中暗适应和光照条件下的双突变体以及玉米中Zm-和Zm-Zm-双突变体的核糖体占据情况。结果表明，RBD1是光诱导翻译所必需的，但对黑暗中的核糖体占据情况影响较小。当不存在HCF136时，RBD1对翻译并非必需，这表明RBD1通过抑制HCF136的抑制作用在光照下激活翻译。相比之下，无论在光照、黑暗还是不存在HCF136的情况下，HCF244都是将核糖体招募到mRNA所必需的。我们进一步证明，翻译激活因子HCF173结合5'UTR并不需要HCF244。这些结果表明，RBD1对于触发D1合成的D1光损伤感知至关重要，并且它通过解除HCF136依赖性组装中间体的抑制来激活翻译。HCF244在这些事件的下游激活，而不影响HCF173与mRNA的结合。结果表明，新生D1的一个受HCF136和RBD1共同影响的特征作为报告D1光损伤的信号，以根据PSII修复的需要调节翻译速率。
