Pentatricopeptide repeat (PPR) proteins are sequence-specific RNA binding proteins located in organelles that are involved in broad range of post-transcriptional RNA maturation processes. The plant PPR proteins are grouped into P and PLS classes which generally fulfil distinct functions. The majority of the PLS-class PPR proteins function as editing factors. P-class PPR proteins act in defining 5' and 3' RNA termini, splicing, and promoting or blocking initiation of translation. Given their role in determining the fate of RNA transcripts, they are good candidates for proteins that can be redesigned for binding RNAs of interest. The recently developed “PPR code” suggests that it should be possible to redesign PPR proteins to recognize particular RNA targets. Based on the code, we are designing libraries of proteins with new RNA specificities. The libraries can have different levels of variability to fit different types and throughput of assays. For example, in-vivo in plants, in-vitro assays, or directed evolution in an emulsion system. In parallel, we are trying to establish new assays for characterizing the binding affinities of PPRs that suit their particular characteristics.