5′ and 3′ ends of chloroplast transcripts can both be stabilised by protein 'caps': a new model for polycistronic RNA maturation

Chloroplast gene expression relies on nucleus-encoded factors acting posttranscriptionally in a gene-specific manner. Among those, RNA stability factors bind to the 5'UTR of their target RNAs to protect them from 5'-> 3' exonucleases. By contrast, little was know, up to now, on the molecular events involved in the complex processing of chloroplast polycistronic transcripts. In this issue of The EMBO Journal, Pfalz et al convincingly demonstrate that PPR10, a maize PPR protein, binds a conserved sequence in the intergenic regions of two distinct polycistronic transcripts. Once bound, PPR10 defines the termini of the processed RNAs issued from these polycistronic precursors by impeding the progression of exonucleases acting from both the 5' and 3' directions. Other PPR proteins likely acting similarly, Barkan and co-workers (Pfalz et al, 2009) propose a new and stimulating model for the maturation of chloroplast transcripts that would involve poorly specific endonucleases and secondary structures or bound proteins that protect transcripts from 5'-> 3' or 3'-> 5' exoribonucleases.