An mTERF domain protein functions in group II intron splicing in maize chloroplasts

The mitochondrial transcription termination factor (mTERF) proteins are nucleic acid binding proteins characterized by degenerate helical repeats of ∼30 amino acids. Metazoan genomes encode a small family of mTERF proteins whose members influence mitochondrial gene expression and DNA replication. The mTERF family in higher plants consists of roughly 30 members, which localize to mitochondria or chloroplasts. Effects of several mTERF proteins on plant development and physiology have been described, but molecular functions of mTERF proteins in plants are unknown. We show that a maize mTERF protein, Zm-mTERF4, promotes the splicing of group II introns in chloroplasts. Zm-mTERF4 coimmunoprecipitates with many chloroplast introns and the splicing of some of these introns is disrupted even in hypomorphic Zm-mterf4 mutants. Furthermore, Zm-mTERF4 is found in high molecular weight complexes that include known chloroplast splicing factors. The splicing of two transfer RNAs (trnI-GAU and trnA-UGC) and one ribosomal protein messenger RNA (rpl2) is particularly sensitive to the loss of Zm-mTERF4, accounting for the loss of plastid ribosomes in Zm-mTERF4 mutants. These findings extend the known functional repertoire of the mTERF family to include group II intron splicing and suggest that a conserved role in chloroplast RNA splicing underlies the physiological defects described for mutations in BSM/Rugosa2, the Zm-mTERF4 ortholog in Arabidopsis.