Polyadenylation and degradation of mRNA in the chloroplast.

Chloroplast development is characterized by the synthesis and assembly of the photosynthetic complexes of the thylakoid membranes. This maturation process requires the coordinated expression of many nuclearand chloroplastencoded genes. As chloroplasts are semiautonomous organelles, they possess their own genome with its inherent transcriptional and translational machinery. However, nuclear-encoded gene products are also necessary for all of the processes occurring in the chloroplast. In this Update we will focus on the control of chloroplast gene expression at the posttranscription stage. Posttranscriptional processes are widely used in controlling the steady-state levels of plastid mRNAs, and are mediated mainly by nuclearencoded proteins, suggesting a way in which the nucleus can modulate gene expression in the chloroplast. For example, mutants of nuclear-encoded genes affecting the accumulation of specific chloroplast transcripts have been described in maize, Arabidopsis, and the green algae Chlamydomonas reinhardtii (Goldschmidt-Clermont, 1998). mRNAs of higher plant and green algae chloroplasts are transcribed as precursor RNAs that undergo a variety of maturation events, including cisand trans-splicing, cleavage of polycistronic messages, processing of 59 and 39 ends, and RNA editing (Fig. 1). A general feature of plastid protein-encoding genes is the presence of inverted repeat sequences in the 39-UTR, which form a stem-loop secondary structure when transcribed to RNA. The 39 end of mature chloroplast mRNAs is located several nucleotides 39 to this stem-loop structure (Fig. 1). Contrary to similar structures found in bacterial mRNA, these elements do not function as transcriptional terminators in chloroplasts; instead, they serve as RNA-processing elements capable of stabilizing upstream RNA fragments in vivo and in vitro (Barkan and Stern, 1998). Different aspects of chloroplast mRNA processing and stability have been the subject of recently published reviews (Barkan and Stern, 1998; Drager and Stern, 1998; Goldschmidt-Clermont, 1998; Nickelsen, 1998) and will not be discussed here. We will focus on recent discoveries concerning the molecular mechanism of mRNA polyadenylation and degradation in the chloroplast, and the proteins involved.