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In mitochondria and chloroplasts of higher plants,the loss of self-splicing activity of degenerate group Ⅱ introns results in recruitment of intron splicing factors encoded in the nucleus.One family of such factors is the pentatricopeptide repeat proteins (PPR) which are implicated in editing,splicing,end maturation and regulation of translation in organellar transcripts.PPR proteins belong to a large family with more than 500 members in flowering plants.Because of its large size and frequent embryo lethality in KO mutants,functions of many PPR proteins remain unknown.Here we report the function identification of PPR17 and PPR20 and their physical interactions with Chloroplast RNA Splicing 1 (CRS1) in maize.Null mutation of PPR17 and PPR20 showed similar arrest in embryogenesis at the transition stage.Both PPR17 and PPR20 are P-type PPR proteins with 11 and 17 PPR motifs respectively.Subcellular localization revealed that both proteins are localized in the chloroplast.PPR17 and PPR20 are expressed in all the tissues examined.Analysis of chloroplast transcripts revealed that loss of either the PPR17 or the PPR20 function abolishes the splicing of the atpF and rpl2 introns,suggesting that PPR17 and PPR20 are required for atpF and rpl2 intron splicing.Previous studies have identified that CRM protein CRS1 is also required for atpF intron splicing (Jenkins et al.,1997;Till et al.,2001).Yeast two hybridization,Pull-down and BiFC analyses indicated that PPR17,PPR20 and CRS1 interact physically.Domain deletion analysis showed that the first four PPR motifs of PPR17 interact with the first two CRM domains of CRS1 and PPR20,and the C-terminal region of CRS1 interacts with PPR20.These results demonstrate that PPR17,PPR20 and CRS1 mediate the atpFintron splicing by forming a complex in maize chloroplasts.