The mitochondrial transcription termination factor family (mTERF) were thought in animal models to be comprised of DNA-binding regulators of mitochondrial transcription. However, are now recognized as important players in organellar gene expression (OGE) not necessarily acting as transcription factors, but rather blocking transcription acting in antisense or by playing a role in ribosomal biogenesis (Kleine et al., 2015). Angiosperms have the highest number of mTERFs out of all eukaryotes and expansion of the family has been proposed to be important to the evolution of land plants (Kleine et al., 2012). For instance, Arabidopsis thaliana has 35 mTERF proteins, 17 of which are mitochondrial targeted, 11 of which are chloroplastic,and 1 that was localized in the cytoplasm (Babiychuk et al., 2011).
So far, it has been experimentally demonstrated that plant mTERFs are required for the transcription termination of chloroplast genes (mTERF6 and mTERF8), transcriptional pausing and the stabilization of chloroplast transcripts (MDA1/mTERF5), intron splicing in chloroplasts (BSM/RUG2/mTERF4 and Zm-mTERF4) and mitochondria (mTERF15 and ZmSMK3) and very recently, also in the assembly of chloroplast ribosomes and translation (mTERF9). (Hammani and Barkan, 2014, Hsu et al., 2014, Robles et al., 2021)
A genome wide survey identified 31 potential mTERF genes in maize (Zhao et al., 2014). Maize mTERF were divided into nine main groups based on phylogenetic analysis, and group IX represented the mitochondria and species-specific clade that diverged from other groups. Tandem and segmental duplication both contributed to the expansion of the mTERF gene family in the maize genome (Zhao et al., 2014).
The maize small kernel 3 (Zmsmk3) gene, encodes a mitochondrial transcription termination factor (mTERF) containing two mTERF motifs, which is conserved in monocots. Reduced expression of ZmSmk3 in a mutant resulted in the splicing deficiency of mitochondrial nad4 intron1 and nad1 intron4, causing a reduction in complex I assembly and activity, impairing mitochondria structure and activating the alternative respiratory pathway and thus contributing to a small kernel phenotype (Pan et al., 2019).
Last updated June 2023 by John Gray
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Pan Z, Ren X, Zhao H, Liu L, Tan Z, Qiu F. A Mitochondrial Transcription Termination Factor, ZmSmk3, Is Required for nad1 Intron4 and nad4 Intron1 Splicing and Kernel Development in Maize. G3 (Bethesda). 2019 Aug 8;9(8):2677-2686. doi: 10.1534/g3.119.400265. PMID: 31196888; PMCID: PMC6686911.