|II.||Expansion of the PPR family||38|
|III.||RNA editing and the evolution of PPR genes||40|
|IV.||Sequence drift in the organelle genomes of early land plants||41|
|V.||Loss of RNA editing in angiosperms||43|
|VI.||Other functions besides editing||44|
The pentatricopeptide repeat (PPR) is a degenerate 35-amino-acid structural motif identified from analysis of the sequenced genome of the model plant Arabidopsis thaliana. From the wealth of sequence information now available from plant genomes, the PPR protein family is now known to be one of the largest families in angiosperm species, as most genomes encode 400–600 members. As the number of PPR genes is generally only c. 10–20 in other eukaryotic organisms, including green algae, the family has obviously greatly expanded during land plant evolution. This provides a rare opportunity to study selection pressures driving a 50-fold expansion of a single gene family. PPR proteins are sequence-specific RNA-binding proteins involved in many aspects of RNA processing in organelles. In this review, we will summarize our current knowledge about the evolution of PPR genes, and will discuss the relevance of the dramatic expansion in the family to the functional diversification of plant organelles, focusing primarily on RNA editing.