| Co-Investigator(Kenkyū-buntansha) |
YAMAO Fumiaki Nagoya Univ. School of Science, Research Assocs. (Present, National Institute of, 助教授 (10158074)
OHAMA Takeshi Nagoya Univ. School of Science, Research Assocs., 理学部, 助手 (00194267)
MUTO Akira Nagoya Univ. School of Science, Associate Profs., 理学部, 助教授 (80034635)
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| Research Abstract |
The prokaryotic genetic code has been influenced by directional mutation pressure (GC/ATpressure) that has been exerted on the entire genome. This pressure affects the synonymous codon choice, the amino composition of proteins and tRNA anticodons.
Unassigned codons would have been produced in bacteria with extremely high GC or AT genomes by deleting certain codons and the corresponding tRNAS. A high AT pressure together with genomic economization led to a change in assignment of the UGA codon, from stop to tryptophan, in Mycoplasma. We studied that in Mycoplasma capricolum, a wall-less gram-positive relative of eubacteria, tRNA^<Arg>_ (as well as the gene for this tRNA), which is needed to read the CGG codon in eubacteria, is absent, and no CGG codons have been found in a total of 6814 codons in M. capricolum genes examined. These results suggest that CGG may be an unassigned (nonsense) codon. To test this, we have constructed a cell-free system of M. capricolum, so as to translate added synthetic mRNA containing in-frame CGG codons. The results show that CGG codon is not translated, and is not used as a termination codon.
The GC content of Micrococcus luteus is about 74%, and no tRNA with anticodon UNN capable of translation codon NNA has been detected, in accordance with a very low or zero usage of NNA codons.
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