The moss Physcomitrella patens, a model plant for the study of RNA editing in plant organelles.

RNA editing is an enigmatic phenomenon in which specific cytidines (C) in the transcripts are changed to uridines (U). In flowering plants, over 500 editing sites have been identified in the mitochondria and plastids. By contrast, in a moss Physcomitrella patens, only 12 editing sites are found in both organelles. Recent extensive genetics studies have revealed involvement of the pentatricopeptide repeat (PPR) proteins with a C-terminal DYW domain (PPR-DYW) in site-specific RNA editing events. Flowering plants have ~100 PPR-DYW genes while P. patens has only 10 PPR-DYW genes. Thus, the number of PPR-DYW genes is somewhat related to the number of RNA editing sites. The P. patens gametophyte, the haploid phase of the life cycle, is dominant, making it possible to study the phenotype of knockouts directly after transformation for gene-targeting. This makes it easier to identify the PPR-DYW proteins required for RNA editing. Recently, we have shown that one of 10 PPR-DYW proteins, PpPPR_71, was responsible for RNA editing in the mitochondrial ccmFc mRNA. In this study, we propose a working model of PpPPR_71 function on RNA editing.