A gene coding for an RPS2 protein is present in the mitochondrial genome of several cereals, but not in dicotyledons

A gene coding for a protein that shows homologies to prokaryotic ribosomal protein S2 is present in the mitochondrial (mt) genome of wheat (Triticum aestivum). The wheat gene is transcribed as a single mRNA which is edited by C-to-U conversions at seven positions, all resulting in alteration of the encoded amino acid. Homologous gene sequences are also present in the mt genomes of rice and maize, but we failed to identify the corresponding sequences in the mtDNA of all dicotyledonous species tested; in these species the mitochondrial RPS2 is probably encoded in the nucleus. The protein sequence deduced from the wheat rps2 gene sequence has a long C-terminal extension when compared to other prokaryotic RPS2 sequences. This extension presents no similarity with any known sequence and is not conserved in the maize or rice mitochondrial rps2 gene. Most probably, after translation, this peptide extension is processed by a specific peptidase to give rise to the mature wheat mitochondrial RPS2.     

A gene coding for an RPS2 protein is present in the mitochondrial genome of several cereals, but not in dicotyledons

A gene coding for a protein that shows homologies to prokaryotic ribosomal protein S2 is present in the mitochondrial (mt) genome of wheat (Triticum aestivum). The wheat gene is transcribed as a single mRNA which is edited by C-to-U conversions at seven positions, all resulting in alteration of the encoded amino acid. Homologous gene sequences are also present in the mt genomes of rice and maize, but we failed to identify the corresponding sequences in the mtDNA of all dicotyledonous species tested; in these species the mitochondrial RPS2 is probably encoded in the nucleus. The protein sequence deduced from the wheat rps2 gene sequence has a long C-terminal extension when compared to other prokaryotic RPS2 sequences. This extension presents no similarity with any known sequence and is not conserved in the maize or rice mitochondrial rps2 gene. Most probably, after translation, this peptide extension is processed by a specific peptidase to give rise to the mature wheat mitochondrial RPS2. Received: 20 November 1997 / Accepted: 29 January 1998     

The wheat cytochrome oxidase subunit II gene has an intron insert and three radical amino acid changes relative to maize

We have determined the sequence of the wheat mitochondrial gene for cytochrome oxidase subunit II (COII) and find that its derived protein sequence differs from that of maize at only three amino acid positions. Unexpectedly, all three replacements are non-conservative ones. The wheat COII gene has a highly-conserved intron at the same position as in maize, but the wheat intron is 1.5 times longer because of an insert relative to its maize counterpart. Hybridization analysis of mitochondrial DNA from rye, pea, broad bean and cucumber indicates strong sequence conservation of COII coding sequences among all these higher plants. However, only rye and maize mitochondrial DNA show homology with wheat COII intron sequences and rye alone with intron-insert sequences. We find that a sequence identical to the region of the 5' exon corresponding to the transmembrane domain of the COII protein is present at a second genomic location in wheat mitochondria. These variations in COII gene structure and size, as well as the presence of repeated COII sequences, illustrate at the DNA sequence level, factors which contribute to higher plant mitochondrial DNA diversity and complexity.     

Sequence analysis of the wheat mitochondrial atp6 gene reveals a fused upstream reading frame and markedly divergent N termini among plant ATP6 proteins

The nucleotide sequence of the wheat mitochondrial gene for subunit 6 (atp6) of the F1F0 ATPase complex has been determined. Unlike bacterial, chloroplast or animal/fungal mitochondrial atp6 counterparts, which encode proteins of about 230-270 amino acids, the wheat mitochondrial atp6 homologue comprises the latter part of an open reading frame (ORF) of 386 codons. The ATP6 protein may therefore by synthesized with a long N-terminal presequence. This is supported by the finding that the ORF is preceded by a conserved sequence block closely related to ones preceding several other actively transcribed wheat mitochondrial protein-coding genes. The fused upstream ORF is similar in length, but unrelated in sequence, to those preceding the maize and tobacco mitochondrial atp6 genes. In wheat, the atp6 gene is located on a recombinationally active repeated DNA element, whose length of 1.4 kb corresponds approximately to that of the atp6 mRNA. A comparison of the wheat and maize ATP6 sequences reveals unexpectedly high divergence in the region corresponding to the mature N-terminal domain and may reflect mitochondrial DNA rearrangements during atp6 gene evolution in monocotyledonous plants.     

Isolation and characterization of cDNAs encoding mitochondrial uncoupling proteins in wheat: wheat UCP genes are not regulated by low temperature

Uncoupling proteins (UCP) found in the inner mitochondrial membrane of mammals dissipate the proton electrochemical gradient across the inner membrane to produce heat rather than synthesize ATP. Using PCR-based methods, we isolated two novel cDNA clones, WhUCP1a and WhUCP1b, that encode the mitochondrial uncoupling protein of wheat (Triticum aestivum L.). The cDNA clones each contain one ORF which can code for a protein of 286 amino acids with a predicted molecular mass of about 30.5 kDa, although three amino acid substitutions are found between them. The deduced amino acid sequences each possess three typical mitochondrial carrier signature domains and six membrane-spanning domains which are highly conserved in the mitochondrial transporter family. Southern analysis suggested that the WhUCP1 gene may be present in as many as three copies in the wheat genome, and also that WhUCP proteins may be encoded by a small multigene family. Northern analysis revealed that the steady-state level of the WhUCP1 mRNA is quite low. Quantitative RT-PCR clearly showed that expression of the WhUCP1 gene in wheat seedlings is insensitive to low temperature. Our data suggest that WhUCP1 might have functions other than low temperature-induced thermogenesis, although WhUCP1 possesses all the typical features reported for known UCPs.     

Structural dynamics of cereal mitochondrial genomes as revealed by complete nucleotide sequencing of the wheat mitochondrial genome

The application of a new gene-based strategy for sequencing the wheat mitochondrial genome shows its structure to be a 452528 bp circular molecule, and provides nucleotide-level evidence of intra-molecular recombination. Single, reciprocal and double recombinant products, and the nucleotide sequences of the repeats that mediate their formation have been identified. The genome has 55 genes with exons, including 35 protein-coding, 3 rRNA and 17 tRNA genes. Nucleotide sequences of seven wheat genes have been determined here for the first time. Nine genes have an exon–intron structure. Gene amplification responsible for the production of multicopy mitochondrial genes, in general, is species-specific, suggesting the recent origin of these genes. About 16, 17, 15, 3.0 and 0.2% of wheat mitochondrial DNA (mtDNA) may be of genic (including introns), open reading frame, repetitive sequence, chloroplast and retro-element origin, respectively. The gene order of the wheat mitochondrial gene map shows little synteny to the rice and maize maps, indicative that thorough gene shuffling occurred during speciation. Almost all unique mtDNA sequences of wheat, as compared with rice and maize mtDNAs, are redundant DNA. Features of the gene-based strategy are discussed, and a mechanistic model of mitochondrial gene amplification is proposed.     

3'-Terminal sequence of wheat mitochondrial 18S ribosomal RNA: further evidence of a eubacterial evolutionary origin.

We have determined the sequences of the 3'-terminal approximately 100 nucleotides of [5' -32P]pCp-labeled wheat mitochondrial, wheat cytosol, and E. coli small sub-unit rRNAs. Sequence comparison demonstrates that within this region, there is a substantially greater degree of homology between wheat mitochondrial 18S and E. coli 16S rRNAs than between either of these and wheat cytosol 18S rRNA. Moreover, at a position occupied by 3-methyluridine in E. coli 16S rRNA, the same (or a very similar) modified nucleoside is present in wheat mitochondrial 18S rRNA but not in wheat cytosol 18S rRNA. Further, E. coli 16S and 23S rRNAs hybridize extensively to wheat mitochondrial 18S and 26S rRNA genes, respectively, but wheat cytosol 18S and 26S rRNAs do not. No other mitochondrial system studies to date has provided comparable evidence that a mitochondrial rRNA is more closely related to its eubacterial homolog than is its counterpart in the cytoplasmic compartment of the same cell. The results reported here provide additional support for the view that plant mitochondria are of endosymbiotic, specifically eubacterial, origin.     

突尼斯两种麦瘿蚊的遗传变异和相关性分析

小麦黑森瘿蚊Mayetiola destructor Say和大麦茎干瘿蚊M. hordei Kieffer是在突尼斯每年都可导致谷物重大损失的两个植食性姊妹种。通常认为为害小麦的瘿蚊是小麦黑森瘿蚊,但是不同谷类物种（小麦或大麦）与麦瘿蚊的这两个种（ destructor 或 hordei ）之间寄主关系并不很严格。提出有效的害虫管理方案首先要求对瘿蚊基因型进行精确分析。本研究应用随机扩增多态DNA（RAPD）技术,结合交配分析和线粒体DNA分型技术,对位于突尼斯北部的一个为害小麦的麦瘿蚊种群的遗传变异程度和分类关联性进行了评估。基于RAPD结果的系统发育分析表明,所研究的种群具有较大的遗传变异范围,这可能源于被分析的小麦样品有被两种瘿蚊共同侵害的复杂背景。虽然交配分析表明有少数不能成功产卵(2/14),但是基于细胞色素b基因限制性酶切分析显示全部样品的线粒体分型均属M. destructor。本文结果进一步证明以RAPD可变性作为分类推断依据不可靠,还为突尼斯M. destructor和M. hordei属于异域分布的观点提供了补充证据。     

Maize and wheat mitochondrial tRNAPro coding regions have similar sequences but different organizations.

The nucleotide sequences of two maize mitochondrial DNA regions containing a tRNAPro gene and an incomplete tRNAPro gene have been compared with the corresponding regions of wheat mitochondrial DNA. These regions have similar sequences but their organization is modified due to different recombination events involving the tRNAPro immediate environment.     

Deletion of the last two exons of the mitochondrialnad7 gene results in lack of the NAD7 polypeptide in aNicotiana sylvestris CMS mutant

InNicotiana sylvestris, two cytoplasmic male sterile (CMS) mutants obtained by protoplast culture show abnormal developmental features of both vegetative and reproductive organs, and mitochondrial gene reorganization following homologous recombination between 65 bp repeated sequences. A mitochondrial region of 16.2 kb deleted from both CMS mutants was found to contain the last two exons of thenad7 gene coding for a subunit of the mitochondrial respiratory chain complex I, which is encoded in the nucleus in fungi and animals but was recently found to be encoded by the mitochondrial genome in wheat. Although theN. sylvestris nad7 gene shows strong homology with its wheat counterpart, it contains only three introns instead of four. Polymerase chain reaction (PCR) experiments indicated that the parental gene organization, including the completenad7 gene, is probably maintained at a substoichiometric level in the CMS mutants, but this proportion is too low to have a significant physiological role, as confirmed by expression studies showing the lack of detectable amounts of the NAD7 polypeptide. Consequently, absence of NAD7 is not lethal to plant cells but a deficiency of complex I could be involved in the abnormal CMS phenotype.