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T Ohama

Publications and source records attributed to T Ohama.

At least 55 records · Page 3Linked to original sources
PubMed1991

Deficiency of apolipoprotein B synthesis in Suncus murinus.

We reported previously that fatty liver is easily induced in a novel experimental animal, Suncus murinus (suncus) by withholding food. In this study, we focused on lipoprotein and apolipoprotein secretion from the liver. The study of lipoproteins from this animal revealed that small amounts of lipoproteins with apolipoprotein (apo) E but without apo B were observed in the fraction of density less than 1.08 g/ml. In order to learn whether apo B is synthesized by the liver or not, isolated suncus livers were perfused with an addition of [35S]methionine. Small amounts of radioactivity were observed in apo E of VLDL, and fairly large amounts in apo E and A-I in the fraction of LDL + HDL, suggesting that VLDL was secreted with apo E but not with apo B from the liver. Northern blot analysis with use of rat apo B cDNA revealed a weak signal of hybridized rat apo B cDNA between 15 kb and 9 kb in the suncus liver and intestinal mucosa; this is almost the same size as rat apo B mRNA. This finding suggests the presence of apo B mRNA in the suncus. In conclusion, apo B is not secreted from the suncus liver, owing to a defect in intracellular post-transcriptional processing or to ineffective transcription. This might be one of the reasons for fatty deposits in the suncus liver. Suncus may be a candidate for an animal model of abetalipoproteinemia as well as fatty liver due to a defect of apo B synthesis.

Animals↗
PubMed1991

Induction of fatty liver by fasting in suncus.

We found that a fatty liver was easily induced in a novel experimental animal, Suncus murinus (suncus), by withholding food. Hepatic triglyceride content increased linearly for up to 24 h after fasting in these animals. Serum levels of neutral lipids are very low in the fed state compared with those in rats, and decreased significantly after 24 h fasting. On the other hand, serum free fatty acids, which are at the same level in fed animals as in rats, increased threefold in the fasting suncus. In order to learn whether the fatty liver induced by fasting is an unusual physiological state or a pathological on-going state in suncus, they were refed after 24 h fasting. Refeeding resulted in a decrease in hepatic triglyceride content to the level of fed animals. Serum lipid levels, which decreased with fasting, returned to those of fed animals. This evidence indicates that hepatic lipid secretion is impaired even in a physiological state to some extent and that starvation causes increasing influx of free fatty acid to the liver, which might be followed by esterification and result in triglyceride accumulation in the liver. In conclusion, hepatic lipid and lipoprotein metabolism is unique to the suncus, which is a useful animal model for the study of intra-hepatic lipid transport.

Animals↗
PubMed1990

Prokaryotic genetic code.

The prokaryotic genetic code has been influenced by directional mutation pressure (GC/AT pressure) that has been exerted on the entire genome. This pressure affects the synonymous codon choice, the amino acid 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.

Anticodon↗
PubMed1990

Evolutionary changes in the genetic code.

The genetic code has been influenced by directional mutation pressure affecting the base composition of DNA, sometimes in the direction of increased GC content and at other times, in the direction of AT. Such pressure led to changes in species-specific usages of codons and tRNA anticodons, and also in amino acid assignments of codons in mitochondria and in several intact organisms. These code changes are probably recent evolutionary events. The genetic code is not 'frozen', but instead it is still evolving.

Base Sequence↗
PubMed1990

Role of GC-biased mutation pressure on synonymous codon choice in Micrococcus luteus, a bacterium with a high genomic GC-content.

The GC (G + C, or G or C)-contents of codon silent positions in all two-codon sets and three codons AUY/A (IIe), and in most of the family boxes of Micrococcus luteus (genomic GC-content: 74%) are 95% to 100% in both the highly and weakly expressed genes. In some family boxes, there is a decrease in NNC codons and an increase in NNG codons from the highly expressed to weakly expressed genes without apparent involvement of NNU and NNA codons. From these observations, we conclude that the selective use of synonymous codons in M. luteus may be largely determined by GC-biased mutation pressure and that in the highly expressed genes tRNAs would act as a weak selection pressure in some family boxes. Available data suggest that the effect of selection pressure by tRNAs on the synonymous codon choice becomes more apparent in the highly expressed genes in eubacteria with intermediate GC-contents such as Escherichia coli and Bacillus subtilis, and that the U/C ratio of the codon third positions in NNU/C-type two-codon sets in the weakly expressed genes would represent the approximate magnitude of directional mutation pressure throughout eubacteria.

Base Sequence↗
PubMed1990

Evolution of the mitochondrial genetic code. III. Reassignment of CUN codons from leucine to threonine during evolution of yeast mitochondria.

Yeast mitochondria use UUR as the sole leucine codons. CUN, universal leucine codons, are read as threonine by aberrant threonine tRNA with anticodon sequence (UAG). The reassignment of CUN codons to threonine during yeast mitochondrial evolution could have proceeded by the disappearance of CUN codons from the reading frames of messenger RNA, through mutation mainly to UUR leucine codons as a result of AT pressure. We suggest that this was accompanied by a loss of leucine-accepting ability of tRNA Leu(UAG). This tRNA could have then acquired threonine-accepting activity through the appearance of an additional threonyl-tRNA synthetase. CUN codons that subsequently appeared from mutations of various other codons would have been translated as threonine. This change in the yeast mitochondrial genetic code is likely to have evolved through a series of nondisruptive nucleotide substitutions that produced no widespread replacement of leucine by threonine in proteins as a consequence.

Adenine↗
PubMed1989

Micrococcus luteus, a bacterium with a high genomic G + C content, contains Escherichia coli-type promoters.

The G + C content of Micrococcus luteus DNA is 74%, which is much higher than that of Escherichia coli (about 50%). In order to understand the influence of GC-directed mutation pressure (GC pressure) on the promoter structure, the initiation sites for both in vitro and in vivo transcription of M. luteus streptomycin (str) and spectinomycin (spc) operons were identified by the reverse transcriptase mapping method. The promoter sequences of M. luteus are similar to those of E. coli, but have significantly higher G + C contents. In an in vitro run-off transcription assay using truncated DNA templates, RNA polymerases from both M. luteus and E. coli were able to transcribe correctly from both the str promoter and the spc major promoter of M. luteus.

Base Sequence↗
PubMed1989

Evolution of the mitochondrial genetic code. I. Origin of AGR serine and stop codons in metazoan mitochondria.

AGA and AGG (AGR) are arginine codons in the universal genetic code. These codons are read as serine or are used as stop codons in metazoan mitochondria. The arginine residues coded by AGR in yeast or Trypanosoma are coded by arginine CGN throughout metazoan mitochondria. AGR serine sites in metazoan mitochondria are occupied mainly in corresponding sites in yeast or Trypanosoma mitochondria by UCN serine, AGY serine, or codons for amino acids other than serine or arginine. Based on these observations, we propose the following evolutionary events. AGR codons became unassigned because of deletion of tRNA Arg (UCU) and elimination of AGR codons by conversion to CGN arginine codons. Upon acquisition by serine tRNA of pairing ability with AGR codons, some codons for amino acids other than arginine mutated to AGR, and were captured by anticodon GCU in serine tRNA. During vertebrate mitochondrial evolution, AGR stop codons presumably were created from UAG stop by deletion of the first nucleotide U and by use of R as the third nucleotide that had existed next to the ancestral UAG stop.

Animals↗
PubMed1989

Evolution of the mitochondrial genetic code. II. Reassignment of codon AUA from isoleucine to methionine.

The reassignment of codon AUA from isoleucine to methionine during mitochondrial evolution may be explained by the codon reassignment (capture) hypothesis without assuming direct replacement of isoleucine by methionine in mitochondrial proteins. According to this hypothesis, codon AUA would have disappeared from the reading frames of messenger RNA. AUA codons would have mutated mainly to AUU isoleucine codons because of constraints resulting from elimination of tRNA Ile with anticodon *CAU (in which *C is lysidine). Later, tRNA Met (CAU) would have undergone structural changes enabling it to pair with both AUG and AUA. AUA codons, formed by mutations of other codons, including AUG, would have reappeared and would have been translated as methionine.

Animals↗
PubMed1989

Spectinomycin operon of Micrococcus luteus: evolutionary implications of organization and novel codon usage.

The complete DNA sequence of the Micrococcus luteus spectinomycin (spc) operon and its adjacent regions has been determined. The sequence has revealed the presence of genes that are homologous to those of the Escherichia coli ribosomal and related proteins, L14, L24, L5, S8, L6, L18, S5, L30, L15, and secretion protein Y (sec Y), and the gene for adenylate kinase (adk). The gene arrangement in the spc operon is essentially the same as that of E. coli except for the absence in the M. luteus spc operon of the genes for S14 and X protein that exist in the E. coli spc operon. SecY and adk seem to be composed of another operon (adk operon) with at least an open reading frame. The deduced amino acid sequences for these ribosomal proteins are well conserved among the two species (40-65% identity). Reflecting the high genomic guanine and cytosine (GC) content of M. luteus (74%), the codon usage of the genes is extremely biased toward use of G and C, about 94% of the codon third positions being G or C. Seven codons, AUA, AAA, AGA, UUA, GUA, CUA, and CAA, all of which have A at the codon third positions, are completely absent in the M. luteus genes examined. Out of 11 genes in the M. luteus spc and adk operons, 5 (10) use GUG (UGA) and 6 (1) use AUG (UAA) as an initiation (termination) codon.

Amino Acid Sequence↗
PubMed1988

Directional mutation pressure and transfer RNA in choice of the third nucleotide of synonymous two-codon sets.

Bacterial species have diverged into a series of families, some with high G + C content in their DNA, and other with high A + T content, resulting, respectively, from G.C- and A.T-directional mutation pressures. Such mutation pressure (G.C/A.T pressure) may be an important determinant for codon usage. It has also been suggested that tRNA acts as a selective constraint for determining codon usage. We have studied the relation between G.C/A.T pressure and tRNA constraints in determining choice of the third nucleotide of eight two-codon sets, using codon usage data obtained from protein genes in four bacterial species, Mycoplasma capricolum, Bacillus subtilis, Escherichia coli, and Micrococcus luteus, and in liverwort (Marchantia polymorpha) chloroplasts. The genomic G + C contents of these range from 25% to 74%. The results demonstrate that tRNA levels act additively to A.T and G.C pressure in affecting contents of A (pairing with *UNN anticodons, in which *U indicates a 2-thiouridine derivative) and C (pairing with GNN anticodons) or G (pairing with CNN anticodons), respectively, in third nucleotide positions of codons.

Chloroplasts↗