Abstract
C-to-U editing is an important event in post-transcriptional RNA processing, which converts a specific cytidine (C)-to-uridine (U) in transcripts of mitochondria and plastids. Typically, the pentatricopeptide repeat (PPR) protein, which specifies the target C residue by binding to its upstream sequence, is involved in the editing of one or a few sites. Here we report a novel PPR-DYW protein EMP21 that is associated with editing of 81 sites in maize. EMP21 is localized in mitochondria and loss of the EMP21 function severely inhibits the embryogenesis and endosperm development in maize. From a scan of 35 mitochondrial transcripts produced by the Emp21 loss-of-function mutant, the C-to-U editing was found to be abolished at five sites (nad7-77, atp1-1292, atp8-437, nad3-275 and rps4-870), while reduced at 76 sites in 21 transcripts. In most cases, the failure to editing resulted in the translation of an incorrect residue. In consequence, the mutant became deficient with respect to the assembly and activity of mitochondrial complexes I and V. As six of the decreased editing sites in emp21 overlap with the affected editing sites in emp5-1, and the editing efficiency at rpl16-458 showed a substantial reduction in the emp21-1 emp5-4 double mutant compared with the emp21-1 and emp5-4 single mutants, we explored their interaction. A yeast two hybrid assay suggested that EMP21 does not interact with EMP5, but both EMP21 and EMP5 interact with ZmMORF8. Together, these results indicate that EMP21 is a novel PPR-DYW protein required for the editing of ~17% of mitochondrial target Cs, and the editing process may involve an interaction between EMP21 and ZmMORF8 (and probably other proteins).
Highlights
Plant mitochondrion possesses its own genome which retains ~5% genes from its prokaryotic ancestor
A novel pentatricopeptide repeat (PPR)-DYW protein EMP21 functions in RNA editing at multiple sites in maize design, data collection and analysis, decision to publish, or preparation of the manuscript
The mutant kernels sampled at 12 days after pollination (DAP) were smaller than the wild type ones, and harbored a very much tiny embryo and a small transparent endosperm while the wild type kernels developed all structures (Fig 1B, 1C, 1E and 1F)
Summary
Plant mitochondrion possesses its own genome which retains ~5% genes from its prokaryotic ancestor. These genes encode proteins, ribosomal RNAs and transfer RNAs for oxidative phosphorylation, and protein translation. The pentatricopeptide repeat (PPR) proteins, which exceed 400 in many species, are one large family of these factors [3,4,5,6]. PPR proteins feature a tandem array of ~35-amino-acid repeat motifs [7], classified into P-class and PLS-class [4]. Many PPR proteins have been identified as being needed for the effective conversion of cytidine to uridine in both the chloroplast and the mitochondrion (reviewed in S3 Table), but the majority of them are responsible for editing at just one or a few sites
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