PPR Proteins Function as a Trans-Factor in Chloroplast RNA Editing

Abstract

In higher plants, RNA editing is a post-transcriptional process of altering a specific C nucleotide to U in an RNA molecule in mitochondria and plastids. For site-specific RNA editing, a cis-element is essential and consists of fewer than thirty nucleotides surrounding the editing site. This cis-element is considered to be a binding site for a trans-factor that may include the editing activity converting C to U. We have identified Arabidopsis crr4 and crr21 mutants specifically defective in the distinct RNA editing events in the plastid ndhD gene. Both genes encode members of the pentatrico-peptide repeat (PPR) protein family, which is extraordinarily large in higher plants. We showed that the recombinant CRR4 binds to the sequence surrounding the editing site. Both CRR4 and CRR21 belong to the E+ subgroup in the PLS subfamily that is characterized by the presence of a conserved C-terminal region. This region is highly conserved and exchangeable between CRR4 and CRR21, although it is not essential for the RNA binding. We conclude that a PPR protein is a trans-factor in the plastid RNA editing and the C-terminal region in the E and E+ subgroups might have a common function among trans-factors.