The P‐class pentatricopeptide repeat protein PpPPR_21 is needed for accumulation of the psbI‐ycf12 dicistronic mRNA in Physcomitrella chloroplasts

Summary Chloroplast gene expression is controlled by numerous nuclear‐encoded RNA‐binding proteins. Among these, pentatricopeptide repeat (PPR) proteins are known to be key players in post‐transcriptional regulation in chloroplasts. However, the functions of many PPR proteins remain unknown. In this study, we characterized the function of a chloroplast‐localized P‐class PPR protein PpPPR_21 in Physcomitrella patens. Knockout (KO) mutants of PpPPR_21 exhibited reduced protonemata growth and lower photosynthetic activity. Immunoblot analysis and blue‐native gel analysis showed a remarkable reduction of the photosystem II (PSII) reaction center protein and poor formation of the PSII supercomplexes in the KO mutants. To assess whether PpPPR_21 is involved in chloroplast gene expression, chloroplast genome‐wide microarray analysis and Northern blot hybridization were performed. These analyses indicated that the psbI‐ycf12 transcript encoding the low molecular weight subunits of PSII did not accumulate in the KO mutants while other psb transcripts accumulated at similar levels in wild‐type and KO mutants. A complemented PpPPR_21KO moss transformed with the cognate full‐length PpPPR_21cDNA rescued the level of accumulation of psbI‐ycf12 transcript. RNA‐binding experiments showed that the recombinant PpPPR_21 bound efficiently to the 5′ untranslated and translated regions of psbImRNA. The present study suggests that PpPPR_21 may be essential for the accumulation of a stable psbI‐ycf12mRNA. Significance Statement Pentatricopeptide repeat (PPR) proteins play important roles in various RNA processing steps in chloroplasts. Here we identified a P‐class PPR protein that is essential for the accumulation of stable psbI‐ycf12 mRNA in chloroplasts of the moss Physcomitrella patens. This PPR protein binds specifically to the 5′ untranslated and translated regions of psbI RNA.