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

Chloroplast gene expression is controlled by numerous nuclear-encoded RNA-binding proteins. Among them, pentatricopeptide repeat (PPR) proteins are known to be key players of posttranscriptional 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 super-complexes 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_21 KO moss transformed with the cognate full-length PpPPR_21 cDNA 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 psbI mRNA. The present study suggests that PpPPR_21 may be essential for the accumulation of a stable psbI-ycf12 mRNA. This article is protected by copyright. All rights reserved.