DER containing two consecutive GTP-binding domains plays an essential role in chloroplast ribosomal RNA processing and ribosome biogenesis in higher plants

This study investigated protein characteristics and physiological functions of DER (Double Era-like GTPase) of higher plants. Nicotiana benthamiana DER (NbDER) contained two tandemly repeated GTP-binding domains (GD) and a C-terminal domain (CTD) that was similar to the K-homology domain involved in RNA binding. Both GDs possessed GTPase activity and contributed to the maximum GTPase activity of NbDER. NbDER fused to green fluorescent protein was localized primarily to chloroplast nucleoids. Arabidopsis der null mutants exhibited an embryonic lethal phenotype, indicating an essential function of DER during plant embryogenesis. Virus-induced gene silencing of NbDER resulted in a leaf-yellowing phenotype caused by disrupted chloroplast biogenesis. NbDER was associated primarily with the chloroplast 50S ribosomal subunit in vivo, and both the CTD and the two GD contributed to the association. Recombinant proteins of NbDER and its CTD could bind to 23S and 16S ribosomal RNAs in vitro. Depletion of NbDER impaired processing of plastid-encoded ribosomal RNAs, resulting in accumulation of the precursor rRNAs in the chloroplasts. NbDER-deficient chloroplasts contained significantly reduced levels of mature 23S and 16S rRNAs and diverse mRNAs in the polysomal fractions, suggesting decreased translation in chloroplasts. These results suggest that DER is involved in chloroplast rRNA processing and ribosome biogenesis in higher plants.