Pentatricopeptide repeat proteins (PPR) are nuclear-encoded proteins and are defined by the tandem array of PPR motif consisting of 35 amino acids. They have been reported to function in post-transcriptional RNA regulation processes such as editing, cleavage and splicing, and in translational regulations in mitochondria or chloroplasts. PPR genes form a huge gene family in higher plants. Bioinformatical analysis has revealed that there are 477 PPR genes in rice and 466 genes in Arabidopsis thaliana (Small and Peters, 2000 Andres et al., 2007 Saha et al., 2007 O’Toole et al., 2008). In Arabidopsis, it was predicted that half of PPRs are targeted to mitochondria, and a quarter may localize in plastids (Lurin et al., 2004). Prediction of organelle targeting has not yet been reported in rice PPR proteins.

To further examine importance of PPR genes for organelle biogenesis and their function, we searched Tos17-insertion lines (Hirochika, 2001) of the 477 PPR genes in rice. By searching a Tos17-flanking DNA sequence database (http://tos.nias.affrc.go.jp/~miyao/pub/tos17/index.html.ja), we identified 30 lines in which Tos17 was inserted in a protein-coding region of 20 PPR genes (Table 1). The database of Tos17 mutant panels has recorded some mutant phenotypes, such as low germination rate, lethal growth, yellow leaf color, and semi-dwarf. Similar phenotypes such as developmental defects (Falcon de Longevialle et al. 2007, 2008), reduced fertility (Lurin et al., 2004) and embryo-lethality (Gutierrez-Marcos et al., 2007) have been reported in PPR mutants of Arabidopsis and maize. Among 29 lines, we identified one homozygous Tos17 inserted line NC0057. This line showed growth retardation. Genetic analysis indicated that this mutation was recessive. We also confirmed linkage between this phenotype and insertion of Tos17. This result lets us expect that at least some of the PPR genes in Table 1 are knocked out by the insertion of Tos17.

Then we predicted subcellular localization of the PPR proteins using four targeting predictors, WoLF PSORT (http://wolfpsort.org/), MITOPROT (http://ihg2.helmholtz-muenchen.de/ihg/mitoprot.html), TargetP 1.1 Server (http://www.cbs.dtu.dk/services/TargetP/) and Predotar (http://urgi.versailles.inra.fr/predotar/predotar.html). Predicted localization is listed in Table 1. Eight proteins were predicted to be targeted to mitochondria, while six were to plastids, by at least three programs. Now we are experimentally analyzing subcelluler localization of the PPR proteins encoded by the 20 genes in Table 1. Detail analyses of these Tos17-insertion lines will reveal essential functions of the PPR genes in mitochondria and plastids in rice.

References
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Falcon de Longevialle A., E. H. Meyer, C. Andres, N. L. Taylor, C. Lurin, A. H. Millar and I. D. Small, 2007. The pentatricopeptide repeat gene OTP43 is required for trans-splicing of the Mitochondrial nad1 intron 1 in Arabidopsis thaliana. Plant Cell 19: 3256-3265.

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