ABSTRACT Nucleus‐encoded proteins are involved in intron splicing in plant chloroplasts. Although many splicing factors were discovered, the mechanisms for chloroplast intron splicing remain unknown. Here, we identified a P‐class pentatricopeptide repeat (PPR) protein, PHOTOSYSTEM ONE BIOGENESIS FACTOR6 (PBF6), that is essential for the accumulation of photosystem I complex. PBF6 bound to ycf3 intron 1 and clpP1 intron 2, and the introns of petB , ndhA , and ndhB transcripts, and was required for their splicing. Truncated PBF6 containing the 10 N‐terminal PPR motifs bound to specific sequences to ycf3 intron 1, clpP1 intron 2, and to the introns of petB , ndhA, and ndhB . PBF6 formed two multi‐subunit splicing complexes with other known splicing factors from the CRM, peptidyl‐tRNA hydrolase, RNase III, PORR, APO, DEAD‐box RNA helicase, and mTERF families. PBF6‐containing complex I was about 600 kDa, comprising nine known splicing factors: CAF1, CAF2, CRS2, CFM3a, RNC1, WTF1, APO2, RH3, and mTERF2. PBF6‐containing complex II was about 300 kDa and contained three known splicing factors: CAF1, CAF2, and CFM2. Furthermore, two known PPR‐type splicing factors, PBF2 and ECD2, both of which are required for splicing the ycf3 intron 1, also formed two multi‐subunit splicing complexes with other known splicing factors. Importantly, these three PPR‐type splicing factors formed their splicing complexes independently. Our data suggest that a PPR‐type splicing factor forms splicing complexes with other known splicing factors under transient expression conditions to facilitate intron splicing and that several PPR‐type splicing factors work together to promote the splicing of the same intron through forming respective splicing complexes.
Li et al. (Thu,) studied this question.