Translation initiation on chloroplast psbA mRNA in plants scales with light intensity, providing its gene product, D1, as needed to replace photodamaged D1 in Photosystem II. The psbA translational activator HIGH CHLOROPHYLL FLUORESCENCE 173 (HCF173) has been hypothesized to mediate this regulation. HCF173 belongs to the short-chain dehydrogenase/reductase superfamily, associates with the psbA 5'-untranslated region (5'-UTR), and has been hypothesized to enhance translation by binding an RNA segment that would otherwise pair with and mask the ribosome binding region. To test these hypotheses, we examined whether a synthetic pentatricopeptide repeat (sPPR) protein can substitute for HCF173 when bound to the HCF173 binding site. We show that an sPPR designed to bind HCF173's footprint in the psbA 5'-UTR bound the intended site in vivo and partially substituted for HCF173 to activate psbA translation. However, sPPR-activated translation did not respond to light. These results imply that HCF173 activates translation, at least in part, by sequestering the RNA it binds to maintain an accessible ribosome binding region, and that HCF173 is also required to regulate psbA translation in response to light. Translational activation can be added to the functions that can be programmed with sPPR proteins for synthetic biology applications in chloroplasts.

中文翻译：

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合成 PPR 蛋白的翻译激活阐明了拟南芥叶绿体中 psbA 翻译的控制

植物中叶绿体 psbA mRNA 的翻译起始随光强度的变化而变化，提供其基因产物 D1，根据需要替换光系统 II 中光损伤的 D1。推测 psbA 翻译激活剂高叶绿素荧光 173 (HCF173) 可以介导这种调节。 HCF173 属于短链脱氢酶/还原酶超家族，与 psbA 5'-非翻译区 (5'-UTR) 相关，并假设通过结合 RNA 片段来增强翻译，否则该 RNA 片段会与核糖体结合配对并屏蔽核糖体结合地区。为了测试这些假设，我们检查了合成的五肽重复序列 (sPPR) 蛋白在与 HCF173 结合位点结合时是否可以替代 HCF173。我们表明，设计用于结合 HCF173 在 psbA 5'-UTR 中的足迹的 sPPR 在体内结合了预期位点，并部分取代了 HCF173 以激活 psbA 翻译。然而，sPPR 激活的翻译对光没有反应。这些结果意味着 HCF173 至少部分地通过隔离其结合的 RNA 以维持可接近的核糖体结合区域来激活翻译，并且 HCF173 还需要响应光来调节 psbA 翻译。可以将翻译激活添加到可用 sPPR 蛋白编程的功能中，以用于叶绿体中的合成生物学应用。