Chloroplast mRNA 3′‐end processing by a high molecular weight protein complex is regulated by nuclear encoded RNA binding proteins.

In the absence of efficient transcription termination correct 3′‐end processing is an essential step in the synthesis of stable chloroplast mRNAs in higher plants. We show here that 3′‐end processing in vitro involves endonucleolytic cleavage downstream from the mature terminus, followed by exonucleolytic processing to a stem‐loop within the 3′‐untranslated region. These processing steps require a high molecular weight complex that contains both endoribonucleases and an exoribonuclease. In the presence of ancillary RNA binding proteins the complex correctly processes the 3′‐end of precursor RNA. In the absence of these ancillary proteins 3′‐end maturation is prevented and plastid mRNAs are degraded. Based on these results we propose a novel mechanism for the regulation of mRNA 3′‐end processing and stability in chloroplasts.