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     

Plant mitochondrial rps2 genes code for proteins with a C-terminal extension that is processed

A gene (rps2) coding for ribosomal protein S2 (RPS2) is present in the mitochondrial (mt) genome of several monocot plants, but absent from the mtDNA of dicots. Confirming that in dicot plants the corresponding gene has been transferred to the nucleus, a corresponding Arabidopsis thaliana nuclear gene was identified that codes for mitochondrial RPS2. As several yeast and mammalian genes coding for mt ribosomal proteins, the Arabidopsis RPS2 apparently has no N-terminal targeting sequence. In the maize mt genome, two rps2 genes were identified and both are transcribed, although at different levels. As in wheat and rice, the maize genes code for proteins with long C-terminal extensions, as compared to their bacterial counterparts. These extensions are not conserved in sequence. Using specific antibodies against one of the maize proteins we found that a large protein precursor is indeed synthesized, but it is apparently processed to give the mature RPS2 protein which is associated with the mitochondrial ribosome.     

Cotranscriptional expression of mitochondrial genes for subunits of NADH dehydrogenase, nad5, nad4, nad2, in Marchantia polymorpha

Three genes for the subunits of the NADH dehydrogenase (nad5, nad4, and nad2) are tandemly clustered on the liverwort mitochondrial genome. Their gene products showed high levels of amino acid sequence identity with the corresponding subunits from higher plant mitochondria (82.8–84.4%), and significant levels of identity with those from liverwort chloroplast (32.0–33.5%), Podospora anserina mitochondria (21.4–45.9%), and human mitochondria (18.4–27.9%). In addition, these three subunits from liverwort mitochondria have conserved amino acid residues in their central regions. The gene nad5 is interrupted by a 672 by group I intron, while genes nad4 and nad2 are interrupted by group II introns of 899 by and 1418 bp, respectively. Northern blot analysis using exon-intron specific probes indicated that these three genes are transcribed as a single precursor mRNA of 9.6 kb in length and are processed into mature mRNA molecules in liverwort mitochondria. Several regions of this nad gene cluster are repeated in the liverwort mitochondrial genome.Communicated by R.G. Herrmann     

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.     

Gene clusters for ribosomal proteins in the mitochondrial genome of a liverwort, Marchantia polymorpha.

We detected 16 genes for ribosomal proteins in the complete sequence of the mitochondrial DNA from a liverwort, Marchantia polymorpha. The genes formed two major clusters, rps12-rps7 and rps10-rpl2-rps19-rps3-rpl16-rpl5- rps14-rps8- rpl6-rps13-rps11-rps1, very similar in organization to Escherichia coli ribosomal protein operons (str and S10-spc-alpha operons, respectively). In contrast, rps2 and rps4 genes were located separately in the liverwort mitochondrial genome (the latter was part of the alpha operon in E. coli). Furthermore, several ribosomal proteins encoded by the liverwort mitochondrial genome differed substantially in size from their counterparts in E. coli and liverwort chloroplast.     

The complete nucleotide sequence and RNA editing content of the mitochondrial genome of rapeseed (Brassica napus L.): comparative analysis of the mitochondrial genomes of rapeseed and Arabidopsis thaliana

The entire mitochondrial genome of rapeseed (Brassica napus L.) was sequenced and compared with that of Arabidopsis thaliana. The 221 853 bp genome contains 34 protein-coding genes, three rRNA genes and 17 tRNA genes. This gene content is almost identical to that of Arabidopsis. However the rps14 gene, which is a pseudo-gene in Arabidopsis, is intact in rapeseed. On the other hand, five tRNA genes are missing in rapeseed compared to Arabidopsis, although the set of mitochondrially encoded tRNA species is identical in the two Cruciferae. RNA editing events were systematically investigated on the basis of the sequence of the rapeseed mitochondrial genome. A total of 427 C to U conversions were identified in ORFs, which is nearly identical to the number in Arabidopsis (441 sites). The gene sequences and intron structures are mostly conserved (more than 99% similarity for protein-coding regions); however, only 358 editing sites (83% of total editings) are shared by rapeseed and Arabidopsis. Non-coding regions are mostly divergent between the two plants. One-third (about 78.7 kb) and two-thirds (about 223.8 kb) of the rapeseed and Arabidopsis mitochondrial genomes, respectively, cannot be aligned with each other and most of these regions do not show any homology to sequences registered in the DNA databases. The results of the comparative analysis between the rapeseed and Arabidopsis mitochondrial genomes suggest that higher plant mitochondria are extremely conservative with respect to coding sequences and somewhat conservative with respect to RNA editing, but that non-coding parts of plant mitochondrial DNA are extraordinarily dynamic with respect to structural changes, sequence acquisition and/or sequence loss.     

Mitochondrial ribosomal protein S9M is involved in male gametogenesis and seed development in Arabidopsis

• Mitochondrial function is critical for cell vitality in all eukaryotes including plants. Although plant mitochondria contain many proteins, few have been studied in the context of plant development and physiology.
• We used knock‐down mutant RPS9M to study its important role in male gametogenesis and seed development in Arabidopsis thaliana.
• Knock‐down of RPS9M in the rps9m‐3 mutant led to abnormal pollen development and impaired pollen tube growth. In addition, both embryo and endosperm development were affected. Phenotype analysis revealed that the rps9m‐3 mutant contained a lower amount of endosperm and nuclear proteins, and both embryo cell division and embryo pattern were affected, resulting in an abnormal and defective embryo. Lowering the level of RPS9M in rps9m‐3 affects mitochondrial ribosome biogenesis, energy metabolism and production of ROS.
• Our data revealed that RPS9M plays important roles in normal gametophyte development and seed formation, possibly by sustaining mitochondrial function.