RESUMEN
Gene expression in plant organelles involves a number of distinct co- or posttranscriptional nucleic acid modifications: 5' and 3' RNA processing, cis- and trans-splicing, RNA stability, and RNA editing. All contribute to the steady-state RNA levels available for the translation of the reduced but essential organellar genetic information. Different from other maturation processes, RNA editing at the transcript level modifies the information encoded by organellar genes and is an essential step for the production of functional proteins. Editing changes are extensive in mitochondria from flowering plants with more than 400 cytidine-to-uridine changes that involve most transcripts, while in chloroplasts they are limited to some RNAs. An additional U-to-C RNA editing reaction is observed with the C-to-U transitions in fern and moss organelles. While RNA editing targets mostly concern coding regions, some events occur in untranslated regions. Whereas RNA editing is genetically and biochemically distinct from other RNA modification activities, evidence is growing for a tight connection between the different processing events. Although the understanding of this astonishing mechanism has increased since its discovery in 1989, some important questions remain unanswered. In this review we discuss the current knowledge on the different aspects of C-to-U, and to a lesser extent U-to-C, and look at RNA editing in plants with a particular emphasis on recent developments involving the role of pentatricopeptide repeat (PPR) proteins in this process.
Asunto(s)
Orgánulos/genética , Plantas/genética , Edición de ARN , ARN de Planta/genética , Orgánulos/metabolismo , Proteínas de Plantas/genética , Proteínas de Plantas/metabolismo , Plantas/metabolismo , ARN de Planta/metabolismoRESUMEN
The Agrocybe aegerita mitochondrial genome possesses two polB genes with linear plasmid origin. The cloning and sequencing of the regions flanking Aa-polB P1 revealed two large inverted repeats (higher than 2421 nt) separated by a single copy region of 5834 nt. Both repeats contain identical copies of the nad4 gene. The single copy region contains two disrupted genes with plasmid origin Aa-polB P1 and a small ORF homologous to a small gene described in two basidiomycete linear plasmids. The phylogenetic analyses argue in favor of a same plasmid origin for both genes but, surprisingly, these genes were separated by a mitochondrial tRNA-Met. Both strands of the complete region containing the two nad4 inverted copies and the tRNA-Met appear to be transcribed on large polycistronic mRNAs. A model summarizing the events that would have occurred is proposed: (1) capture of the tRNA by the plasmid before its integration in the mtDNA or acquisition of the tRNA gene by recombination after the plasmid integration, (2) integration of the plasmid in the mtDNA, accompanied by a large duplication containing the nad4 gene and (3) erosion of the plasmid sequences by large deletions and mutations.