A family of RRM-type RNA-binding proteins specific to plant mitochondria

Abstract

Expression of higher plant mitochondrial (mt) genes is regulated at the transcriptional, posttranscriptional, and translational levels, but the vast majority of the mtDNA and RNA-binding proteins involved remain to be identified. Plant mt single-stranded nucleic acid-binding proteins were purified by affinity chromatography, and corresponding genes have been identified. A majority of these proteins belong to a family of RNA-binding proteins characterized by the presence of an N-terminal RNA-recognition motif (RRM) sequence. They diverge in their C-terminal sequences, suggesting that they can be involved in different plant mt regulation processes. Mitochondrial localization of the proteins was confirmed both in vitro and in vivo and by immunolocalization. Binding experiments showed that several proteins have a preference for poly(U)-rich sequences. This mt protein family contains the ubiquitous RRM motif and has no known mt counterpart in non-plant species. Phylogenetic and functional analysis suggest a common ancestor with RNA-binding glycine-rich proteins (GRP), a family of developmentally regulated proteins of unknown function. As with several plant, cyanobacteria, and animal proteins that have similar structures, the expression of one of the Arabidopsis thaliana mt RNA-binding protein genes is induced by low temperatures.H igher plants require mitochondrial (mt) function for their survival, which depends on proper mtDNA maintenance and expression (1). At the structural level, the mtDNA of plants is relatively large, and in most species it is constantly reorganized by recombination between repeated sequences (2). Although large portions of mtDNA have been moved around during evolution, the plant mt genome evolves very slowly through nucleotide substitution; plant mt gene sequences have remained remarkably constant, suggesting the existence of very efficient DNA repair systems. On the other hand, the expression of plant mt genes is also complexly regulated, both at the transcriptional, posttranscriptional, and translational levels. For proper maturation of its transcripts, mitochondria puts to play complex processes of intron splicing, 5Ј and 3Ј RNA trimming, extensive RNA editing by C to U conversions, and regulation of transcript stability by secondary structures and polyadenylation (3, 4). Despite the importance of these mt processes in plant development, little is known about the factors involved, but at the core of the protein complexes must be DNA-binding proteins involved in mtDNA replication, recombination, repair and transcription, and RNA-binding proteins involved in posttranscriptional RNA maturation and translation. As a first step in the dissection of these complexes, we undertook to purify and identify nucleic acid-binding proteins from plant mitochondria. Most of the proteins identified are plant-specific, and many belong to a previously undescribed family of mt RNA-binding proteins (mRBP) characterized by the presence of an N-terminal RNA recognition motif (RRM). This family of plant mRBPs is ph...

An RNA recognition motif-containing protein is required for plastid RNA editing in Arabidopsis and maize

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“…CP, chloroplast ribonucleoprotein; PABP, poly(A) binding protein; snRP, small nuclear ribonucleoprotein. In the ORRM1 clade, the figure in parenthesis is the annotation (s) for small given in Lorkovic and Barta (23), or (m) for mitochondrial given in Vermel et al (24). showed a reduction or a lack of editing extent in the mutant exhibited a significant increase of their editing extent in the transfected protoplasts with the full-length ORRM1 (Fig.…”

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“…Section 1734 solely to indicate this fact. microscope observation were as previously described (16).…”