The P‐class pentatricopeptide repeat protein Pp PPR _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 Pp PPR _21 in Physcomitrella patens . Knockout ( KO ) mutants of Pp PPR _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 Pp PPR _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 Pp PPR _21 KO moss transformed with the cognate full‐length Pp PPR _21 cDNA rescued the level of accumulation of psbI ‐ ycf12 transcript. RNA ‐binding experiments showed that the recombinant Pp PPR _21 bound efficiently to the 5′ untranslated and translated regions of psbI mRNA . The present study suggests that Pp PPR _21 may be essential for the accumulation of a stable psbI‐ycf12 mRNA .