Abstract
Present-day chloroplast and mitochondrial genomes contain only a few dozen genes involved in ATP synthesis, photosynthesis, and gene expression. The proteins encoded by these genes are only a small fraction of the many hundreds of proteins that act in chloroplasts and mitochondria. Hence, the vast majority, including components of organellar gene expression (OGE) machineries, are encoded by nuclear genes, translated into the cytosol and imported to these organelles. Consequently, the expression of nuclear and organellar genomes has to be very precisely coordinated. Furthermore, OGE regulation is crucial to chloroplast and mitochondria biogenesis, and hence, to plant growth and development. Notwithstanding, the molecular mechanisms governing OGE are still poorly understood. Recent results have revealed the increasing importance of nuclear-encoded modular proteins capable of binding nucleic acids and regulating OGE. Mitochondrial transcription termination factor (mTERF) proteins are a good example of this category of OGE regulators. Plant mTERFs are located in chloroplasts and/or mitochondria, and have been characterized mainly from the isolation and analyses of Arabidopsis and maize mutants. These studies have revealed their fundamental roles in different plant development aspects and responses to abiotic stress. Fourteen mTERFs have been hitherto characterized in land plants, albeit to a different extent. These numbers are limited if we consider that 31 and 35 mTERFs have been, respectively, identified in maize and Arabidopsis. Notwithstanding, remarkable progress has been made in recent years to elucidate the molecular mechanisms by which mTERFs regulate OGE. Consequently, 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). This review aims to provide a detailed update of current knowledge about the molecular functions of plant mTERF proteins. It principally focuses on new research that has made an outstanding contribution to unravel the molecular mechanisms by which plant mTERFs regulate the expression of chloroplast and mitochondrial genomes.
Highlights
The gene functions that persist in the chloroplast and mitochondria genomes are essential for the activity of these organelles and, for plant life
Given the marked genomic erosion suffered by the genomes of the endosymbionts from which chloroplasts and mitochondria originate, organellar gene expression (OGE) control is principally performed by proteins encoded by nuclear genes
We have reviewed the works that mainly contributed to shed light on plant Mitochondrial transcription termination factor (mTERF) molecular functions
Summary
Current OGE apparatuses exhibit a mixture of gene expression features from prokaryotes, eukaryotes, and new innovations [6] This is especially relevant in chloroplast transcriptional machinery because chloroplasts utilize a functional, multi-subunit prokaryotic-type polymerase (PEP), having core subunits encoded by the organellar genome and accessory subunits encoded by the nuclear genome, including proteins with DNA/RNA binding domains. These mutant analyses have been instrumental for shedding light on the function of mTERF in plants, and in animals (reviewed in [31]) Along these lines, plant mterf defective mutants usually exhibit stunted growth and paleness due to low chlorophyll levels when mTERFs are located in chloroplasts, and altered transcript abundance of chloroplast and/or mitochondrial genes. The aforementioned mutant analyses reveal that mTERFs play a fundamental role in plant growth and development, and in their response and adaptation to adverse environmental conditions (recently reviewed in [44])
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