Group II intron splicing factors in plant mitochondria

Gregory G Brown,Catherine Colas Des Francs-Small,Oren Ostersetzer-Biran

doi:10.3389/fpls.2014.00035

Abstract

Group II introns are large catalytic RNAs (ribozymes) which are found in bacteria and organellar genomes of several lower eukaryotes, but are particularly prevalent within the mitochondrial genomes (mtDNA) in plants, where they reside in numerous critical genes. Their excision is therefore essential for mitochondria biogenesis and respiratory functions, and is facilitated in vivo by various protein cofactors. Typical group II introns are classified as mobile genetic elements, consisting of the self-splicing ribozyme and its intron-encoded maturase protein. A hallmark of maturases is that they are intron specific, acting as cofactors which bind their own cognate containing pre-mRNAs to facilitate splicing. However, the plant organellar introns have diverged considerably from their bacterial ancestors, such as they lack many regions which are necessary for splicing and also lost their evolutionary related maturase ORFs. In fact, only a single maturase has been retained in the mtDNA of various angiosperms: the matR gene encoded in the fourth intron of the NADH-dehydrogenase subunit 1 (nad1 intron 4). Their degeneracy and the absence of cognate ORFs suggest that the splicing of plant mitochondria introns is assisted by trans-acting cofactors. Interestingly, in addition to MatR, the nuclear genomes of angiosperms also harbor four genes (nMat 1-4), which are closely related to maturases and contain N-terminal mitochondrial localization signals. Recently, we established the roles of two of these paralogs in Arabidopsis, nMAT1 and nMAT2, in the splicing of mitochondrial introns. In addition to the nMATs, genetic screens led to the identification of other genes encoding various factors, which are required for the splicing and processing of mitochondrial introns in plants. In this review we will summarize recent data on the splicing and processing of mitochondrial introns and their implication in plant development and physiology, with a focus on maturases and their accessory splicing cofactors.

Highlights

PLANT MITOCHONDRIAL GENOMES Mitochondria in plants house the oxidative phosphorylation (OXPHOS) machinery and many other essential metabolic pathways
The vast majority of the proteins responsible for these processes, as well as those that participate in the biogenesis of the organelle are encoded in the nuclear genome
The mtDNAs in plants encode tRNAs, rRNAs, ribosomal proteins, subunits of the respiratory machinery, including NADH:ubiquinone oxidoreductase, the cytochrome bc1 complex, cytochrome c oxidase, ATP-synthase, and several other proteins involved in cytochrome c biogenesis and the twin-arginine protein translocation (Unseld et al, 1997; Kubo et al, 2000; Adams et al, 2002; Notsu et al, 2002; Handa, 2003; Clifton et al, 2004; Ogihara et al, 2005; Sugiyama et al, 2005)

Summary

Introduction

PLANT MITOCHONDRIAL GENOMES (mtDNAs) Mitochondria in plants house the oxidative phosphorylation (OXPHOS) machinery and many other essential metabolic pathways (for review see Millar et al, 2011). Four other maturase-related proteins, denoted as nMAT 1–4, are encoded by nuclear genes, but have N-terminal mitochondrial targeting sequences (Keren et al, 2009).

Results

Conclusion