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Mechanistic insights into PPR proteins

The PPR motif is a degenerate 35 amino acid repeat that is believed to belong to the larger family of helical repeat motifs (Small and Peeters, 2000). These typically bind macromolecules through a surface formed by the stacking of consecutive helical repeats. Several PPR proteins have been genetically-characterized, and all of these influence RNA metabolism, suggesting that PPR proteins bind RNA. Indeed, several PPR proteins exhibit RNA binding activity in vitro (Nakamura et al., 2003; Kazama et al., 2008; Williams-Carrier et al., 2008). PPR proteins have a broad phylogenetic distribution, but the family is highly expanded specifically in the plant lineage: ~450 PPR proteins are annotated in Arabidopsis, and rice genomes (O'Toole et al., 2008). The majority of PPR proteins are predicted to be targeted to mitochondria and chloroplasts (Lurin et al., 2004). For large protein families, functional redundancy is a common theme, but surprisingly, analyses of insertional mutations in PPRs have revealed that a large fraction of PPR genes is essential for plant development, as evidenced by their homozygous mutants being embryo-lethal in Arabidopsis (Lurin et al., 2004; Cushing et al., 2005). A subset of PPR proteins has been demonstrated to stabilize mRNA and at the same time foster translation of these same mRNAs. We are interested tp understand this dual function using genetic and biochemical approaches.