| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 1990: ¥300,000 (Direct Cost: ¥300,000)
Fiscal Year 1989: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 1988: ¥1,000,000 (Direct Cost: ¥1,000,000)
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| Research Abstract |
Structural alterations of the chloroplasts genome tend to occur at "hot spots" on the physical map. To clarify the mechanism of mutation of chloroplasts genome structure in higher plants, we determined the nucleotide sequence of the hot-spot region of chloroplast DNAs related to length mutations (deletions/insertions) in Triticum (wheat) and Aegilops. From a comparison of this region in wheat with the corresponding region of tobacco, rice or linverwort, it is evident that one of the open reading frames in tobacco (ORF512) has been replaced in wheat and rice by the rp123 gene, which is a member of the ribosomal protein gene operon. In the deleted positions and in the original genome of Triticum and Aegilops, consensus sequences forming short direct repeats were found, indicating that these deletions were a result of intramolecular recombination mediated by these short direct-repeat sequences. By two independent recombination events in the Aegilops crassa type of chloroplast genome, whic
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h is shared by Triticum monococcum, Ae. Bicornis, Ae. Sharonensis, Ae. Comosa, and Ae. Mutica, the novel chloroplast DNA sequences of T. Aestivum and Ae. Squarrosa were generated. This finding indicates the existence of illegitimate recombination in the chloroplast genome and presents a mechanism for producing genetic diversity of that genome. DNA segments homologous with ORF512 were found only in the mitochondrial genome of Ae. Crassa, and DNA homology with unique sequence in the wheat chloroplast DNA was detected in the nucleus DNA as repetitive sequence. The facts indicate genetic flux among organellar genomes.
Nucleotide divergence of chloroplast DNAs around the hot spot region related to length mutation was also analyzed. The evolution of these genes located in the hot spot region among closely related wheat complex is characterized by nonbiased nucleotide substitutions, A-T pressure, transitions over transversions, and frequent changes at the third codon position, in contrast with the gene evolution among more distant plant groups. Assuming 60 million years as the divergence time between wheat and maize, the divergence within wheat groups, the divergence of wheat from barley, and that from rice are estimated based on the sequence homology to be 1.5, 10, and 40 million years, respectively. Less
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