Abstract
Expression of most plastid genes involves multiple post-transcriptional processing events, such as splicing, editing, and intercistronic processing. The latter involves the formation of mono-, di-, and multicistronic transcripts, which can further be regulated by differential stability and expression. The plastid pentacistronic psbB transcription unit has been well characterized in vascular plants. It encodes the subunits CP47 (psbB), T (psbT), and H (psbH) of photosystem II as well as cytochrome b 6 (petB) and subunit IV (petD) of the cytochrome b 6 f complex. Each of the petB and petD genes contains a group II intron, which is spliced during post-transcriptional modification. The small subunit of photosystem II, PsbN, is encoded in the intercistronic region between psbH and psbT but is transcribed in the opposite direction. Expression of the psbB gene cluster necessitates different processing events along with numerous newly evolved specificity factors conferring stability to many of the processed RNA transcripts, and thus exemplarily shows the complexity of RNA metabolism in the chloroplast.
Highlights
The chloroplast evolved as a result of an endosymbiotic event in which a cyanobacterial ancestor was taken over by a eukaryotic cell
Each of the photosystem II as well as cytochrome b6 (petB) and petD genes contains a group II intron, which is spliced during post-transcriptional modification
A highly sophisticated system of transcript maturation including endo- and exonucleolytic activities, splicing, editing, and modulation of RNA stability has been developed which is not exploited to the same extent in the free-living cyanobacterial ancestor
Summary
The chloroplast evolved as a result of an endosymbiotic event in which a cyanobacterial ancestor was taken over by a eukaryotic cell. Various mechanisms can determine the stability of chloroplast mRNAs, including protection of RNA termini by proteins or RNA secondary structures. Numerous nuclear-encoded factors have been acquired for processing and other post-transcriptional modifications of plastid transcripts (Stern et al 2010; Barkan 2011). Most if not all protein-coding genes on vascular plant chloroplasts are found in polycistronic transcription units Their intercistronic processing can differ between plant species and results in complex transcript pattern creating mono-, di-, and multicistronic transcripts which can further be regulated by differential stability. Evolved specificity factors confer stability to many of these RNA transcripts by binding to their termini and blocking exoribonucleases. The evolution of the psbB cluster genes (Fig. 1), along with functions of the encoded proteins and known factors for transcript processing and stability events (Fig. 2) will be elaborated in this review
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