RNA Editing of Cytochrome Oxidase Subunit III in Sunflower Mitochondria

Abstract

Direct sequencing of cytochrome oxidase subunit IlIl (coxIll) mRNA with a specific primer confirms RNA editing in sunflower (Helianthus annus) mitochondria. Six instances of mRNA editing could be verified, one of these specific to this species. All the editing events involve C to U transitions in the coxill mRNA causing codon changes that lead to amino acids better conserved in evolution than those encoded in the genomic DNA. This observation confirms RNA editing to be widespread in higher plant mitochondria.analyses confirmed mRNA editing alterations at the protein level, this process seems to be essential for the correct expression of plant mitochondrial protein genes (1, 7). Recently, nucleotide and amino acid sequence comparisons of the positions shown to be edited in Oenothera (11) and wheat (9) coxIII2 cDNA led us to predict that editing is also required in sunflower (Helianthus annuus Gloriasol) mitochondria to synthesize an evolutionarily conserved, functional COXIII ( 16). To test these predictions we determined the sequence of a selected region in the coxlll mRNA from sunflower mitochondria by reverse transcriptase sequencing with a specific primer.The term RNA editing is used to describe any process that results in the production of a messenger RNA with a nucleotide sequence different from its own DNA template. RNA editing seems to be operative in a wide range of biological systems likely as a specific control point of gene expression, though both the nature of the modification and the mechanisms proposed vary considerably.The phenomenon was first discovered in the mitochondria of kinetoplastid trypanosomes (2, 18) where the mRNAs contain insertions or deletions of uridine residues relative to the sequence of their genes. Soon thereafter other examples of RNA editing were reported (reviewed in refs. 3 and 19): (a) a single precise, developmentally regulated cytosine to uridine change in mammalian apolipoprotein B mRNA; (b) insertion of one or more guanosine residues in paramyxovirus P mRNA; and (c) most recently, insertion of cytosine residues in Physarum polycephalum mitochondrial mRNAs (14).RNA editing has also been described for several proteincoding genes in higher plant mitochondria (see ref. 20 for a review). All editing events described so far in the plant species investigated, primarily two monocots (wheat, maize) and two dicots (Oenothera, Petunia), involve a individual nucleotide alteration modifying the genome-encoded cytidines to uridines (4, 5, 8, l 1) and in a few cases the genomic thymidines to cytidines (9, 17). Because most of the nucleotide conversions result in modifications of the codons which then specify amino acids that are conserved in the mitochondrial proteins of non-plant organisms, it appear that RNA editing in plant mitochondria plays a role in the conservation of protein sequences during evolution (8).Moreover, as amino acid composition and peptide sequence 'Supported by the Ministero della Ricerca Scientifica e Tecnologica (M.U.R.S.T., 60% and 40% Funds). 1261 ...

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“…Saiardi and Quagliariello [28] revealed that direct sequencing of cytochrome oxidase subunit III (coxIII) mRNA with a specific primer confirms RNA editing in sunflower (Helianthus annus) mitochondria. Six instances of mRNA editing could be verified, one of these specific to this species.…”