In the green alga Chlamydomonas (Chlamydomonas reinhardtii), chloroplast gene expression is tightly regulated posttranscriptionally by gene-specific trans-acting protein factors. Here, we report the identification of the octotricopeptide repeat protein MTHI1, which is critical for the biogenesis of chloroplast ATP synthase oligomycin-sensitive chloroplast coupling factor. Unlike most trans-acting factors characterized so far in Chlamydomonas, which control the expression of a single gene, MTHI1 targets two distinct transcripts: it is required for the accumulation and translation of atpH mRNA, encoding a subunit of the selective proton channel, but it also enhances the translation of atpI mRNA, which encodes the other subunit of the channel. MTHI1 targets the 5' untranslated regions of both the atpH and atpI genes. Coimmunoprecipitation and small RNA sequencing revealed that MTHI1 binds specifically a sequence highly conserved among Chlorophyceae and the Ulvale clade of Ulvophyceae at the 5' end of triphosphorylated atpH mRNA. A very similar sequence, located ~60 nucleotides upstream of the atpI initiation codon, was also found in some Chlorophyceae and Ulvale algae species and is essential for atpI mRNA translation in Chlamydomonas. Such a dual-targeted trans-acting factor provides a means to coregulate the expression of the two proton hemi-channels.

在绿藻衣藻（Chlamydomonas r einhardtii）中，叶绿体基因表达在转录后受到基因特异性反式蛋白因子的严格调控。在这里，我们报告鉴定的八肽重复蛋白MTHI1，这对于叶绿体ATP合酶寡聚敏感的叶绿体偶联因子的生物发生至关重要。不像大多数反式作用因子，其特征衣藻，其中控制单基因的表达，到目前为止，MTHI1靶向两种不同的转录物：它需要的积累和翻译atpH的mRNA，编码选择性质子通道的一个亚基，但它还增强了atpI的翻译mRNA，编码通道的另一个亚基。MTHI1针对两者的5'非翻译区atpH和ATPI基因。免疫共沉淀和小分子RNA测序表明，MTHI1在三磷酸化的atpH mRNA的5'末端特异地结合了在绿藻科和Ulphyphyce的Ulvale进化枝之间高度保守的序列。在一些绿藻科和Ulvale藻类物种中也发现了一个非常相似的序列，位于atpI起始密码子上游〜60个核苷酸处，对于衣藻中atpI mRNA的翻译至关重要。这种双重靶向的反式作用因子提供了一种使两个质子半通道的表达相互关联的手段。