THE SEQUENCE AND STRUCTURE OF THE 3'-UNTRANSLATED REGIONS OF CHLOROPLAST TRANSCRIPTS ARE IMPORTANT DETERMINANTS OF MESSENGER-RNA ACCUMULATION AND STABILITY

A general characteristic of the S'-untranslated regions (3' UTRs) of p lastid mRNAs is an inverted repeat (IR) sequence that can fold into a stem-loop structure. These stem-loops are RNA 3'-end processing signal s and determinants of mRNA stability, not transcription terminators. I ncubation of synthetic RNAs corresponding to the 3' UTRs of Chlamydomo nas chloroplast genes atpB and petD with a chloroplast protein extract resulted in the accumulation of stable processing products. Synthetic RNAs of the petA 3' UTR and the antisense strand of atpB 3' UTR were degraded in the extract. To examine 3' UTR function in vivo, the atpB 3' UTR was replaced with the 3' UTR sequences of the Chlamydomonas chl oroplast genes petD, petD plus trnR, rbcL, petA and E. coli thrA by bi olistic transformation of Chlamydomonas chloroplasts. Each 3' UTR was inserted in both the sense and antisense orientations. The accumulatio n of both total atpB mRNA and ATPase beta-subunit protein in all trans formants was increased compared to a strain in which the atpB 3' UTR h ad been deleted. However, the level of discrete atpB transcripts in tr ansformants containing the antisense 3' UTR sequences was reduced to a pproximately one-half that of transformants containing the 3' UTRs in the sense orientation. These results imply that both the nucleotide se quences and the stem-loop structures of the 3' UTRs are important for transcript 3'-end processing, and for accumulation of the mature mRNAs .