Pentatricopeptide repeat (PPR) proteins are RNA binding proteins with functions in organelle RNA metabolism. They are found in all eukaryotes but have been most extensively studied in plants. We report on the identification of 12 PPR-encoding genes in the genome of the protist Dictyostelium discoideum, with potential homologs in other members of the same lineage and some predicted novel functions for the encoded gene products in protists. For one of the gene products, we show that it localizes to the mitochondria, and we also demonstrate that antisense inhibition of its expression leads to slower growth, a phenotype associated with mitochondrial dysfunction. 1. Introduction Mitochondria contain their own genome and, as is the case for any other genome, must maintain tight control over the expression of their encoded gene products. Mitochondrial genes typically encode either components of the respiratory chain for ATP synthesis or the mitochondrial translation machinery. Regulating the expression of such genes is therefore essential for normal cell function, as aberrations in the regulation of mitochondrial gene expression can result in disease [1, 2]. Similarly to nuclear and bacterial gene expression, post-transcriptional regulation is one of the most important stages of mitochondrial gene expression. This can include processing of polycistronic transcripts and liberation of structural RNAs, excision of introns, RNA editing, and stability modifications such as polyadenylation [2]. Given that these post-transcriptional processes are highly diverse, one would expect such functions to be catalysed by many different proteins. Indeed, each post-transcriptional event often involves several proteins, amongst which a large family of helical repeat proteins have been found to play important roles in organelle gene expression. These rather complex proteins are known as pentatricopeptide repeat (PPR) proteins and were originally identified during the sequencing of the genome of the model plant Arabidopsis thaliana [3]. The PPR family is now known as one of the largest protein families to exist in angiosperms with over 450 PPR-encoding genes identified in A. thaliana [4]. PPR proteins are characterised by a 35 amino acid motif, often repeated in tandem a variable number of times [3, 5]. Each PPR motif consists of two antiparallel α-helices, which interact with each other [3, 5]. The series of α-helices form a superhelix containing a groove, which can bind its RNA ligand in a sequence-specific manner [5–7]. Most PPR proteins function as molecular adaptors in the
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