![]() ![]() These are only a couple of examples, and for more discussion of the molecular functions of PPR proteins readers are redirected to reviews elsewhere that treat this topic comprehensively ( Andres et al., 2007 Delannoy et al., 2007 Saha et al., 2007 Schmitz-Linneweber & Small, 2008 Chateigner-Boutin & Small, 2010). Arabidopsis CRR4 was the first gene found in plants to be directly involved in cytosine to uridine RNA editing ( Kotera et al., 2005), since followed by many other PPR editing factors ( Table 1). For example, CRP1 in maize is shown to associate with the 5′UTR region of photosynthetic genes psaC and petA, and mutants disrupted in CRP1 gene lack translation of these genes, leading to the defects in photosynthesis ( Fisk et al., 1999 Schmitz-Linneweber et al., 2005). The general picture that results from these studies is that PPR proteins form sequence-specific associations with RNA, and that these associations affect folding, processing and/or translation of the RNA, thus manipulating expression of the transcript. Since the discovery and definition of the PPR consensus sequence ( Small & Peeters, 2000), predicted to form an antiparallel double alpha-helical motif, many studies have been conducted on PPR proteins covering biochemistry, molecular functions, cellular functions and roles in development ( Schmitz-Linneweber & Small, 2008). Subsequently, Pet309, as well as the protein P67 implicated in transcription in Triticum aestivum mitochondria ( Ikeda & Gray, 1999) and CRP1 in Zea mays involved in translation of photosynthesis genes ( Fisk et al., 1999 Schmitz-Linneweber et al., 2005), were recognized to be members of a large family of related proteins following the systematic analysis of the Arabidopsis thaliana (thale cress) genome ( Aubourg et al., 2000 Small & Peeters, 2000). The first PPR protein to be described was the Saccharomyces cerevisiae mitochondrial protein Pet309, found to participate in translation of cox1 ( Manthey & McEwen, 1995 Manthey et al., 1998 Tavares-Carreon et al., 2008). Pentatricopeptide repeat (PPR) proteins are the most numerous of these. ![]() RNA metabolism plays a particularly important role in organelle gene expression ( Stern et al., 2010) and a wide array of different RNA binding proteins are found in organelles. As plant mitochondria encode no machinery to manage their own RNA expression and post-transcriptional RNA modification processes, these essential steps are totally reliant on nuclear-encoded gene products ( Binder & Brennicke, 2003). Although these represent only a small proportion of all mitochondrial proteins, their importance to mitochondrial function means that incorrect regulation of these mitochondrial genes would severely affect the whole system. 40 proteins are encoded within the mitochondrial genome, most of which encode essential subunits of oxidative phophorylation enzymatic complexes and ribosomal proteins ( Kubo et al., 2000 Notsu et al., 2002 Handa, 2003 Ogihara et al., 2005 Sugiyama et al., 2005 Tian et al., 2006 Allen et al., 2007 Kubo & Newton, 2008 Fujii et al., 2010). For example, the plant mitochondrial proteome can be estimated to consist of c. Many of these functions are carried out by enzyme complexes with both organelle-encoded and nucleus-encoded subunits, and therefore coordination of the expression of organelle and nuclear genes is a critical matter. Organellar function is essential for eukaryotic life, and depends upon activities that are maintained by proteins either internally synthesized within the organelle (organelle-encoded) or imported from cytosol (nucleus-encoded). In this review, we will summarize our current knowledge about the evolution of PPR genes, and will discuss the relevance of the dramatic expansion in the family to the functional diversification of plant organelles, focusing primarily on RNA editing. PPR proteins are sequence-specific RNA-binding proteins involved in many aspects of RNA processing in organelles. This provides a rare opportunity to study selection pressures driving a 50-fold expansion of a single gene family. 10–20 in other eukaryotic organisms, including green algae, the family has obviously greatly expanded during land plant evolution. As the number of PPR genes is generally only c. From the wealth of sequence information now available from plant genomes, the PPR protein family is now known to be one of the largest families in angiosperm species, as most genomes encode 400–600 members. The pentatricopeptide repeat (PPR) is a degenerate 35-amino-acid structural motif identified from analysis of the sequenced genome of the model plant Arabidopsis thaliana. ![]()
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