Two novel PLS-class pentatricopeptide repeat proteins are involved in the group II intron splicing of mitochondrial transcripts in the moss Physcomitrella patens

Pentatricopeptide repeat (PPR) proteins are RNA binding proteins that function in posttranscriptional regulation as gene-specific regulators of RNA metabolism in plant organelles. Plant PPR proteins are divided into four classes: P, PLS, E and DYW. The E- and DYW-class proteins are mainly implicated in RNA editing, whereas most of the P-class proteins predominantly participate in RNA cleavage, splicing and stabilization. In contrast, the functions of PLS-class proteins still remain obscure. Here, we report the function of PLS-class PpPPR_31 and PpPPR_9 in Physcomitrella patens. The knockout (KO) mutants of PpPPR_31 and PpPPR_9 exhibited slower protonema growth compared to the wild type. The PpPPR_31 KO mutants showed a considerable reduction in the splicing of nad5 intron 3 and atp9 intron 1. The PpPPR_9 KO mutants displayed severely reduced splicing of cox1 intron 3. An RNA electrophoresis mobility shift assay showed that the recombinant PpPPR_31 protein bound to the 5' region of nad5 exon 4 and the bulged-A region in domain VI of atp9 group II intron 1 while the recombinant PpPPR_9 bound to the translated region of ORF622 in cox1 intron 3. These results suggest that PLS-class PPR proteins may influence the splicing efficiency of mitochondrial group II introns. To characterize the function of a mitochondria-localized P-class PPR protein PpPPR_31 in Physcomitrella patens, gene expression of organellar genome of 4 day old protonema cells of PpPPR_31 KO mutants and wild type were measured.