CHLOROPLAST MESSENGER-RNA 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 p rocessing is an essential step in the synthesis of stable chloroplast mRNAs in higher plants, We show here that 3'-end processing in vitro i nvolves endonucleolytic cleavage downstream from the mature terminus, followed by exonucleolytic processing to a stem-loop within the 3'-unt ranslated region. These processing steps require a high molecular weig ht complex that contains both endoribonucleases and an exoribonuclease . In the presence of ancillary RNA binding proteins the complex correc tly processes the 3'-end of precursor RNA. In the absence of these anc illary proteins 3'-end maturation is prevented and plastid mRNAs are d egraded. Based on these results we propose a novel mechanism for the r egulation of mRNA 3'-end processing and stability in chloroplasts.