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We have determined the complete nucleotide sequence of chloroplast DNA from a liverwort, Marchantia polymorpha, using a clone bank of chloroplast DNA fragments. The circular genome consists of 121,024 base-pairs and includes two large inverted repeats (IRA and IRB, each 10,058 base-pairs), a large single-copy region (LSC, 81,095 base-pairs), and a small single-copy region (SSC, 19,813 base-pairs). The nucleotide sequence was analysed with a computer to deduce the entire gene organization, assuming the universal genetic code and the presence of introns in the coding sequences. We detected 136 possible genes. 103 gene products of which are related to known stable RNA or protein molecules. Stable RNA genes for four species of ribosomal RNA and 32 species of tRNA were located, although one of the tRNA genes may be defective. Twenty genes encoding polypeptides involved in photosynthesis and electron transport were identified by comparison with known chloroplast genes. Twenty-five open reading frames (ORFs) show structural similarities to Escherichia coli RNA polymerase subunits, 19 ribosomal proteins and two related proteins. Seven ORFs are comparable with human mitochondrial NADH dehydrogenase genes. A computer-aided homology search predicted possible chloroplast homologues of bacterial proteins; two ORFs for bacterial 4Fe-4S-type ferredoxin, two for distinct subunits of a protein-dependent transport system, one ORF for a component of nitrogenase, and one for an antenna protein of a light-harvesting complex. The other 33 ORFs, consisting of 29 to 2136 codons, remain to be identified, but some of them seem to be conserved in evolution. Detailed information on gene identification is presented in the accompanying papers. We postulated that there were 22 introns in 20 genes (8 tRNA genes and 12 ORFs), which may be classified into the groups I and II found in fungal mitochondrial genes. The structural gene for ribosomal protein S12 is trans-split on the opposite DNA strand. The universal genetic code was confirmed by the substitution pattern of simultaneous codons, and by possible codon recognition of the chloroplast-encoded tRNA molecules, assuming no importation of tRNA molecules from the cytoplasm. The nucleotide residue A or T is preferred at the third position of the codons (G+C, 11.9%) and in intergenic spacers (G+C, 19.5%), resulting in an overall G+C content that is low (28.8%) throughout the liverwort chloroplast genome. Possible gene expression signals such as promoters and terminators for transcription, predicted locations of gene products, and DNA replicative origins are discussed.
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PMID:Structure and organization of Marchantia polymorpha chloroplast genome. I. Cloning and gene identification. 246 54

A region of about 2 kb which is almost identical in the wheat and maize mitochondrial genomes has been sequenced. It contains a tRNA(Ser) gene, a pseudo-tRNA gene and two open reading frames coding for subunit 3 of the NADH dehydrogenase (118 amino acids) and for ribosomal protein S12 (125 amino acids). The two protein genes are separated by 47 bp and are co-transcribed in wheat and maize. Two transcripts of about 0.9 kb and 3.0 kb, each coding for both proteins, have been characterized, but no monocistronic transcript was detected. Each gene is preceded by a putative ribosome binding site. The pseudo-tRNA gene is interrupted by two insertion sequences in wheat and by one in maize. The origin of the additional interrupting sequence found in the wheat pseudo-tRNA gene, which is also present elsewhere in the mitochondrial genomes, is discussed.
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PMID:The genes coding for subunit 3 of NADH dehydrogenase and for ribosomal protein S12 are present in the wheat and maize mitochondrial genomes and are co-transcribed. 285 27

In the present study, the intergenic region between the mitochondrial genes encoding subunit 3 of NADH dehydrogenase (nad3) and ribosomal protein S12 (rps12) was shown to contain a Gn mononucleotide microsatellite repeat. This region was analysed in 15 species belonging to the genus Pinus and interspecific variation was detected in the form of repeat length polymorphism. Sequence analysis of a 576-bp region containing the microsatellite confirmed that the variability was due to expansion and contraction of the repeat motif and that no point mutations were present in the coding regions of the two genes. This is the first report of the occurrence of a microsatellite polymorphism in plant mitochondria.
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PMID:An example of microsatellite length variation in the mitochondrial genome of conifers. 1020 8

Trypanosoma brucei possesses a highly complex RNA editing system that uses guide RNAs to direct the insertion and deletion of uridines in mitochondrial mRNAs. These changes extensively alter the target mRNAs and can more than double them in length. Recently, analyses showed that several of the edited genes possess the capacity to encode two different protein products. The overlapped reading frames can be accessed through alternative RNA editing that shifts the translated reading frame. In this study, we analyzed the editing patterns of three putative dual-coding genes, ribosomal protein S12 (RPS12), the 5' editing domain of NADH dehydrogenase subunit 7 (ND7 5'), and C-rich region 3 (CR3). We found evidence that alternatively 5'-edited ND7 5' and CR3 transcripts are present in the transcriptome, providing evidence for the use of dual ORFs in these transcripts. Moreover, we found that CR3 has a complex set of editing pathways that vary substantially between cell lines. These findings suggest that alternative editing can work to introduce genetic variation in a system that selects against nucleotide mutations.
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PMID:Cell-line specific RNA editing patterns in Trypanosoma brucei suggest a unique mechanism to generate protein variation in a system intolerant to genetic mutations. 3184 Jan 76