MDA1, a nucleus‐encoded factor involved in the stabilization and processing of the atpA transcript in the chloroplast of Chlamydomonas

Summary In Chlamydomonas reinhardtii, chloroplast gene expression is tightly regulated post‐transcriptionally by gene‐specific trans‐acting protein factors. Here, we report the molecular identification of an OctotricoPeptide Repeat (OPR) protein, MDA1, which governs the maturation and accumulation of the atpA transcript, encoding subunit α of the chloroplast ATP synthase. As does TDA1, another OPR protein required for the translation of the atpA mRNA, MDA1 targets the atpA 5’‐untranslated region (UTR). Unexpectedly, it binds within a region of approximately 100 nt in the middle of the atpA 5’‐UTR, at variance with the stabilization factors characterized so far, which bind to the 5’‐end of their target mRNA to protect it from 5’ → 3’ exonucleases. It binds the same region as TDA1, with which it forms a high‐molecular‐weight complex that also comprises the atpA mRNA. This complex dissociates upon translation, promoting degradation of the atpA mRNA. We suggest that atpA transcripts, once translated, enter the degradation pathway because they cannot reassemble with MDA1 and TDA1, which preferentially bind to de novo transcribed mRNAs. Significance Statement Two OctotricoPeptide Repeat (OPR) proteins, MDA1 and TDA1, respectively, required for the accumulation and translation of the atpA mRNA interact with a region in the middle of the atpA 5’‐UTR, instead of the usual interaction with its 5’‐end, to form a ternary complex. This complex dissociates upon translation, which promotes the degradation of the atpA mRNA.