The P-class pentatricopeptide repeat protein binds to the 5’ untranslated and translated regions of chloroplast psbI-ycf12 transcript and is involved in mRNA stability

Chloroplast gene expression is controlled by numerous nuclear-encoded RNA-binding proteins. Among them, pentatricopeptide repeat (PPR) proteins are known to be a key player in 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 a reduced growth of the protonemata and lower photosynthetic activity. Immuno-blot analysis and blue-native gel analysis showed a remarkable reduction of the photosystem II (PSII) reaction center protein and poorly formation of the PSII super-complexes in the KO mutants. To access whether PpPPR_21 is involved in the 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 of WT and the KO mutants. A complemented PpPPR_21 KO moss transformed with the cognate full-length PpPPR_21 cDNA rescued the psbI transcript accumulation level. RNA binding experiments showed that the recombinant PpPPR_21 bound efficiently to the 5’-untraslated and translated region of the psbI mRNA. The present study suggests that PpPPR_21 may be essential for accumulation of a stable psbI-ycf12 mRNA. To characterize the function of a chloroplast-localized P-class PPR protein PpPPR_21 in Physcomitrella patens, gene expression of organellar genome of 4 day old protonema cells of PpPPR_21 KO mutants and wild type were measured.