The OPR Protein MTHI1 Controls the Expression of Two Different Subunits of ATP Synthase CFo in <i>Chlamydomonas reinhardtii</i>

Abstract

In the green alga Chlamydomonas (<i>Chlamydomonas r</i> <i>einhardtii</i>), chloroplast gene expression is tightly regulated posttranscriptionally by gene-specific <i>trans</i>-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 <i>trans</i>-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 <i>atpH</i> mRNA, encoding a subunit of the selective proton channel, but it also enhances the translation of <i>atpI</i> mRNA, which encodes the other subunit of the channel. MTHI1 targets the 5' untranslated regions of both the <i>atpH</i> and <i>atpI</i> 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 <i>atpH</i> mRNA. A very similar sequence, located ∼60 nucleotides upstream of the <i>atpI</i> initiation codon, was also found in some Chlorophyceae and Ulvale algae species and is essential for <i>atpI</i> mRNA translation in Chlamydomonas. Such a dual-targeted <i>trans</i>-acting factor provides a means to coregulate the expression of the two proton hemi-channels.