The functions of many plant genes remain unknown even in the extensively studied model plant Arabidopsis. Photosynthesis and respiration rely on the expression of genes in chloroplasts and mitochondria. Gene expression in plant organelles is particularly complex and requires the participation of nuclear encoded factors, which are imported into the organelles. Genetic and bioinformatic analyses suggest that many hundreds of nuclear genes influence organellar gene expression. The proteins of the mTERF family are specific to metazoan and plants, and are all predicted to localize to mitochondria or chloroplasts. Whereas the mTERF family is small (3-4 members) in animals and regulates gene expression in mitochondria, about 30 members are found in higher plants but their molecular functions and biochemical properties are largely unexplored. We will test the hypothesize that members of this family influence mitochondria and chloroplast gene expression by interacting with RNA or DNA in organelles. We will use Arabidopsis and maize reverse-genetics, in vitro nucleic-acid binding assays, and a coimmunoprecipitation assay to identify the native RNA or DNA ligands of representative members of the mTERF family in plants.