Short non-coding RNA fragments accumulating in chloroplasts: footprints of RNA binding proteins?

Chloroplast RNA metabolism is controlled and excecuted by hundreds of nuclear-encoded, chloroplast-localized RNA binding proteins. Contrary to the nucleo-cytosolic compartment or bacteria, there is little evidence for non-coding RNAs that play a role as riboregulators of chloroplasts. We mined deep-sequencing datasets to identify short (16-28 nt) RNAs in the chloroplast genome and found 50 abundant small RNAs (sRNAs) represented by multiple, in some cases, thousands of sequencing reads, whereas reads are in general absent from the surrounding sequence space. Other than sRNAs representing the most highly abundant mRNAs, tRNAs and rRNAs, most sRNAs are located in non-coding regions and many are found a short distance upstream of start codons. By transcript end mapping we show that the 5' and 3' termini of chloroplast RNAs coincide with the ends of sRNAs. Sequences of sRNAs identified in Arabidopsis are conserved between different angiosperm species and in several cases, we identified putative orthologs in rice deep sequencing datasets. Recently, it was suggested that small chloroplast RNA fragments could result from the protective action of pentatricopeptide repeat (PPR) proteins against exonucleases, i.e. footprints of RNA binding proteins. Our data support this scenario on a transcriptome-wide level and suggest that a large number of sRNAs are in fact remnants of PPR protein targets.