OCCURRENCE OF PLASTID RNA EDITING IN ALL MAJOR LINEAGES OF LAND PLANTS

RNA editing changes posttranscriptionally single nucleotides in chloro plast-encoded transcripts, Although much work has been done on mechani stic and functional aspects of plastid editing, little is known about evolutionary aspects of this RNA processing step, To gain a better und erstanding of the evolution of RNA editing in plastids, we have invest igated the editing patterns in ndhB and rbcL transcripts from various species comprising all major groups of land plants, Our results indica te that RNA editing occurs in plastids of bryophytes, fern allies, tru e ferns, gymnosperms, and angiosperms. Both editing frequencies and ed iting patterns show a remarkable degree of interspecies variation, Fur thermore, we have found that neither plastid editing frequencies nor t he editing pattern of a specific transcript correlate with the phyloge netic tree of the plant kingdom, The poor evolutionary conservation of editing sites among closely related species as well as the occurrence of single species-specific editing sites suggest that the differences in the editing patterns and editing frequencies are probably due both to independent loss and to gain of editing sites, In addition, our re sults indicate that RNA editing is a relatively ancient process that p robably predates the evolution of land plants, This supposition is in good agreement with the phylogenetic data obtained for plant mitochond rial RNA editing, thus providing additional evidence for common evolut ionary roots of the two plant organellar editing systems.