[PPR proteins--modular factors regulating expression of organellar genomes].

PPR proteins make up the most numerous family of RNA-binding proteins identified to date. They localize almost exclusively to plastids and mitochondria of eukaryotic organisms. The most striking feature of this family is the expansion of PPR protein-encoding genes in vascular plants, which likely coincided with plants colonizing land. PPR proteins participate in stabilizing, editing, splicing, degradation and processing of policistronic transcripts, as well as translation activation in mitochondria and plastids. Although the number of PPR proteins in non-plant organisms is significantly smaller than in plants, they still play a crucial role in regulating the expression of mtDNA. Disruptions of PPR protein-encoding genes usually result in severe phenotypic consequences. Plant PPR proteins bind RNA in a sequence-specific manner, where a single PPR motif recognizes an individual nucleotide in a given sequence. This opens up possibilities for engineering de novo synthetic protein sequences that would interact with precisely determined organellar sequences, thus enabling modulation of mtDNA and ctDNA expression.