The Organelle RNA Recognition Motif (RRM) Proteins in Plants

The RRM domain is a structurally conserved region of 80-90 amino acids, defined by two short consensus sequences, RNP1 (hexapeptide) and RNP2 (octapeptide). The RRM domain is highly plastic, binding to RNAs with a wide range of affinities and sequence-specificities and also interacts with diverse protein domains. The Arabidopsis thaliana genome encodes 196 RRM-containing proteins. Despite the efforts of many laboratories, the function of many RRM proteins is still largely unknown. Although plant RRM proteins are encoded by the nuclear genome, some RRM proteins are imported into plant chloroplasts or mitochondria. Twenty-three Arabidopsis RRM proteins have been reported to be located in chloroplasts, whereas ten have been predicted to be found in the mitochondrion by screening published databases and by using protein subcellular prediction tools. Organellar RRM proteins have been shown to participate in various aspects of RNA metabolism including RNA editing, splicing and stability, as well as translation . They are also involved in plant development and stress responses, functioning as protein or RNA chaperones in some conditions. Our group has characterized two chloroplast-targeted and 4 mitochondrial-targeted RRM proteins, identifying 5 of them as RNA editing factors. Mutations in some organelle RRM proteins affect both RNA editing and environmental stress response and flowering time, indicating that the proteins may perform multiple functions in the plant.