Polyadenylation and Degradation of mRNA in the Chloroplast

Abstract

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 [1–4]. 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 [5], Arabidopsis [6] and the green alga Chlamydomonas reinhardtii, reviewed in [4, 7]. 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 [8–10]. Instead, they serve as RNA-processing elements in vitro and are capable of stabilizing upstream RNA fragments in vivo and in vitro [8, 9, 11–14]. Here we focus on recent discoveries concerning the molecular mechanism of mRNA degradation in the chloroplast and the proteins involved.