Abstract
Following the endosymbiotic acquisition of mitochondria by eukaryotic cells, most of the genes in this organelle were transferred to the nucleus. To maintain mitochondrial biogenesis and function, nuclear and mitochondrial genomes require regulated and coordinated expression. In plant organelles, nuclear-encoded proteins targeted to the organelles control posttranscriptional and posttranslational mechanisms. Pentatricopeptide repeat (PPR) proteins are good candidates to play such regulatory roles. Here, we identify PNM1 (for PPR protein localized to the nucleus and mitochondria 1), a novel PPR protein that is dual localized to mitochondria and nuclei in Arabidopsis thaliana, as observed by green fluorescent protein fusions and immunodetection on subcellular fractions and on histological sections. Genetic complementation showed that loss of PNM1 function in mitochondria, but not in nuclei, is lethal for the embryo. In mitochondria, it is associated with polysomes and may play a role in translation. A genetic screen in yeast identified protein partners of PNM1. These partners, the nucleosome assembly protein NAP1, and the transcription factor TCP8 interact with PNM1 in the nucleus in planta. Furthermore, TCP8 can bind the promoter of PNM1. This suggests that PNM1 might be involved in the regulation of its own gene expression in the nucleus and could thus play a role in gene expression adjustments between mitochondria and the nucleus.