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Sökning: L773:0028 0836 OR L773:1476 4687 > (1990-1999) > (1998)

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  • Andersson, Siv GE, et al. (författare)
  • The genome sequence of Rickettsia prowazekii and the origin of mitochondria
  • 1998
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 396:6707, s. 133-140
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>We describe here the complete genome sequence (1,111,523 base pairs) of the obligate intracellular parasite Rickettsia prowazekii, the causative agent of epidemic typhus. This genome contains 834 protein-coding genes. The functional profiles of these genes show similarities to those of mitochondrial genes: no genes required for anaerobic glycolysis are found in either R. prowazekii or mitochondrial genomes, but a complete set of genes encoding components of the tricarboxylic acid cycle and the respiratory-chain complex is found in R. prowazekii. In effect, ATP production in Rickettsia is the same as that in mitochondria. Many genes involved in the biosynthesis and regulation of biosynthesis of amino acids and nucleosides in free-living bacteria are absent from R. prowazekii and mitochondria. Such genes seem to have been replaced by homologues in the nuclear (host) genome. The R. prowazekii genome contains the highest proportion of non-coding DNA (24%) detected so far in a microbial genome. Such non-coding sequences may be degraded remnants of 'neutralized' genes that await elimination from the genome. Phylogenetic analyses indicate that R. prowazekii is more closely related to mitochondria than is any other microbe studied so far.</p>
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  • Billker, Oliver, et al. (författare)
  • Identification of xanthurenic acid as the putative inducer of malaria development in the mosquito
  • 1998
  • Ingår i: Nature. - Macmillan Publishers Ltd.. - 0028-0836 .- 1476-4687. ; 392:6673, s. 289-292
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>Malaria is transmitted from vertebrate host to mosquito vector by mature sexual blood-living stages called gametocytes. Within seconds of ingestion into the mosquito bloodmeal, gametocytes undergo gametogenesis. Induction requires the simultaneous exposure to at least two stimuli in vitro: a drop in bloodmeal temperature to 5 degrees C below that of the vertebrate host, and a rise in pH from 7.4 to 8.0-8.2. In vivo the mosquito bloodmeal has a pH of between 7.5 and 7.6. It is thought that in vivo the second inducer is an unknown mosquito-derived gametocyte-activating factor. Here we show that this factor is xanthurenic acid. We also show that low concentrations of xanthurenic acid can act together with pH to induce gametogenesis in vitro. Structurally related compounds are at least ninefold less effective at inducing gametogenesis in vitro. In Drosophila mutants with lesions in the kynurenine pathway of tryptophan metabolism (of which xanthurenic acid is a side product), no alternative active compound was detected in crude insect homogenates. These data could form the basis of the rational development of new methods of interrupting the transmission of malaria using drugs or new refractory mosquito genotypes to block parasite gametogenesis.</p>
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  • Chen, Richard Z, et al. (författare)
  • DNA hypomethylation leads to elevated mutation rates
  • 1998
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 395:6697, s. 89-93
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>Genome-wide demethylation has been suggested to be a step in carcinogenesis. Evidence for this notion comes from the frequently observed global DNA hypomethylation in tumour cells, and from a recent study suggesting that defects in DNA methylation might contribute to the genomic instability of some colorectal tumour cell lines. DNA hypomethylation has also been associated with abnormal chromosomal structures, as observed in cells from patients with ICF (Immunodeficiency, Centromeric instability and Facial abnormalities) syndrome and in cells treated with the demethylating agent 5-azadeoxycytidine. Here we report that murine embryonic stem cells nullizygous for the major DNA methyltransferase (Dnmt1) gene exhibited significantly elevated mutation rates at both the endogenous hypoxanthine phosphoribosyltransferase (Hprt) gene and an integrated viral thymidine kinase (tk) transgene. Gene deletions were the predominant mutations at both loci. The major cause of the observed tk deletions was either mitotic recombination or chromosomal loss accompanied by duplication of the remaining chromosome. Our results imply an important role for mammalian DNA methylation in maintaining genome stability.</p>
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  • Kanopka, Arvydas, et al. (författare)
  • Regulation of adenovirus alternative RNAsplicing by dephosphorylation of SR proteins
  • 1998
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 393:6681, s. 185-187
  • Tidskriftsartikel (refereegranskat)abstract
    • <p>SR proteins are a family of essential splicing factors required for early recognition of splice sites during spliceosome assembly. They also function as alternative RNA splicing factors when overexpressed in vivo or added in excess to extracts in vitro. SR proteins are highly phosphorylated in vivo, a modification that is required for their function in spliceosome assembly and splicing catalysis. Here we show that SR proteins purified from late adenovirus-infected cells are inactivated as splicing enhancer or splicing repressor proteins by virus-induced dephosphorylation. We further show that the virus-encoded protein E4-ORF4 activates dephosphorylation by protein phosphatase 2A of HeLa SR proteins and converts their splicing properties into that of SR proteins purified from late adenovirus-infected cells. Taken together, our results suggest that E4-ORF4 is an important factor controlling the temporal shift in adenovirus alternative RNA splicing. We conclude that alternative pre-mRNA splicing, like many other biological processes, is regulated by reversible protein phosphorylation.</p>
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  • Näsholm, Torgny, et al. (författare)
  • Boreal forest plants take up organic nitrogen
  • 1998
  • Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 392:6679, s. 914-916
  • Tidskriftsartikel (populärvet., debatt m.m.)abstract
    • <p>Plant growth in the boreal forest, the largest terrestrial biome, is generally limited by the availability of nitrogen. The presumed cause of this limitation is slow mineralization of soil organic nitrogen<sup>1</sup>,<sup>2</sup>. Here we demonstrate, to our knowledge for the first time, the uptake of organic nitrogen in the field by the trees <em>Pinus sylvestris</em> and <em>Picea abies</em>, the dwarf shrub <em>Vaccinium myrtillus</em> and the grass <em>Deschampsia flexuosa</em>. These results show that these plants, irrespective of their different types of root–fungal associations (mycorrhiza), bypass nitrogen mineralization. A trace of the amino acid glycine, labelled with the stable isotopes <sup>13</sup>C and <sup>15</sup>N, was injected into the organic (mor) layer of an old successional boreal coniferous forest. Ratios of <sup>13</sup>C:<sup>15</sup>N in the roots showed that at least 91, 64 and 42% of the nitrogen from the absorbed glycine was taken up in intact glycine by the dwarf shrub, the grass and the trees, respectively. Rates of glycine uptake were similar to those of <sup>15</sup>N-ammonium. Our data indicate that organic nitrogen is important for these different plants, even when they are competing with each other and with non-symbiotic microorganisms. This has major implications for our understanding of the effects of nitrogen deposition, global warming and intensified forestry.</p>
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