In the chloroplasts of higher plant and green algae mRNAs are transcribed as precursor RNAs which undergo a variety of maturation events, including cis-and trans-splicing, cleavage of polycistronic messages, processing of 5' and 3' ends, and RNA editing. These post-transcriptional processes play an important role in regulating gene expression and in the chloroplast, they may be controlled in part by nuclear encoded-genes. For example, mutants of nuclear-encoded genes which affect the accumulation of specific chloroplast transcripts have been characterized in maize, Arabidopsis and the green alga Chlamydomonas reinhardtii, reviewed in. A general characteristic of the plastid protein-coding region is the presence of inverted-repeat sequences in the 3' untranslated region (UTR), which form a stem-loop structure when transcribed to RNA. The 3' end of mature chloroplast mRNAs is located several nucleotides 3' to this stem-loop structure. In contrast to similar structures found in bacterial mRNA, in chloroplasts these elements do not function as transcriptional terminators. Instead, they serve as RNA-processing elements in vitro and are capable of stabilizing upstream RNA fragments in vivo and in vitro. Here we focus on recent discoveries concerning the molecular mechanism of mRNA degradation in the chloroplast and the proteins involved.
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