Abstract
In plants, splicing of organellar group II introns involves numerous nucleus-encoded trans-factors. But, how these trans-factors function and interact is not well understood. Here we report the function of a pentatricopeptide repeat (PPR) protein PPR14 and its physical relationship with other splicing factors in mitochondria. Null mutations of PPR14 severely arrest the embryo and endosperm development, causing an empty pericarp phenotype. PPR14 is required for the splicing of NADH dehydrogenase 2 (nad2) intron 3 and nad7 introns 1 and 2 in mitochondria. The absence of nad2 and nad7 transcripts leads to disruption of the mitochondrial complex I assembly and abolishes its NADH dehydrogenase activity. This is accompanied with increased levels of other mitochondrial complexes and elevated expression of the alternative oxidase proteins. As the function of PPR14 overlaps with PPR-SMR1 and the CRM-domain containing protein Zm-mCSF1, we tested their interactions. Protein-protein interaction analysis indicated that PPR14 interacts with PPR-SMR1 and Zm-mCSF1, suggesting that these three proteins may form a complex. As PPR proteins and CRM-domain containing proteins have many members in mitochondria and chloroplasts, we propose that organellar group II intron splicing is probably mediated by a dynamic complex that includes different PPR and CRM proteins in plants.
Highlights
In higher plants, most genes of the endosymbiont are either lost or transferred to nucleus during evolution (Martin, 2003; Huang et al, 2005)
PPR14 Is a Canonical P-Type pentatricopeptide repeat (PPR) Protein Targeted to Mitochondria
Phylogenetic analysis of PPR14 orthologs from 24 representative plant species which are classified as the lineages of monocots and eudicots shows that monocots and eudicots form two separate clades, and the maize PPR14 protein is conserved in monocots (Figure 1C)
Summary
Most genes of the endosymbiont are either lost or transferred to nucleus during evolution (Martin, 2003; Huang et al, 2005). The mitochondrial genome only retains about 60 identified genes. The maize mitochondrial genome was reported to include 58 identified genes encoding 3 ribosomal RNAs (rRNAs), 22 tRNAs and 33 known proteins involved in respiratory chain directly or indirectly (Clifton et al, 2004). The mitochondrial genome contains 22 group II introns. The majority are cis-splicing introns, which are excised from single precursor RNAs. Six of them are transsplicing introns and widely scattered in the genome along with their up- or down-stream exons due to mitochondrial DNA rearrangements. Six of them are transsplicing introns and widely scattered in the genome along with their up- or down-stream exons due to mitochondrial DNA rearrangements These exon-intron segments are transcribed independently and exons are ligated together through transsplicing
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