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排序方式: 共有383条查询结果,搜索用时 31 毫秒
1.
I Baeza P Gariglio L M Rangel P Chavez L Cervantes C Arguello C Wong C Monta?ez 《Biochemistry》1987,26(20):6387-6392
2.
B Singer F Chavez S J Spengler J T Ku?mierek L Mendelman M F Goodman 《Biochemistry》1989,28(4):1478-1483
The effect of alkyl group size on ability to act as deoxythymidine triphosphate (dTTP) has been studied for the carcinogen products O2-methyl-, O2-ethyl-, and O2-isopropyl-dTTP by using three types of nucleic acids as template and DNA polymerase I (Pol I) or Klenow fragment as the polymerizing enzymes. Apparent Km and relative Vmax values were determined in primer extension on M13 DNA at a single defined site, in poly[d(A-T)], and in nicked DNA. These data are the basis for calculation of the relative rate of insertion opposite A, relative to dTTP. The insertion rate for any O2-alkyl-dTTP is much higher than for a mismatch between unmodified dNTPs. Unexpectedly, O2-isopropyl-dTTP is more efficiently utilized than O2-methyl-dTTP or O2-ethyl-dTTP on each of the templates. O2-isopropyl-dTTP also substitutes for dTTP over extended times of DNA synthesis at a rate only slightly lower than that of dTTP. Parallel experiments using O4-methyl-dTTP under the same conditions show that it is incorporated opposite A more frequently than is O2-methyl-dTTP. Therefore, both the ring position and the size of the alkyl group influence polymerase recognition. Once formed, all O2-alkyl-T.A termini permit elongation, as does O4-methyl-T.A. In contrast to the relative difficulty of incorporating the O-alkyl-dTTPs, formation of the following normal base pair (C.G) occurs rapidly when dGTP is present. This indicates that a single O-alkyl-T.A pair does not confer significant structural distortion recognized by Pol I. 相似文献
3.
4.
Cody Raul Cardenas Andrew J. Mularo Andreas S. Chavez Rachelle M. M. Adams 《Biotropica》2023,55(1):145-159
The coevolutionary relationship between fungus-growing ants (Formicidae: Attini: Attina) and their symbionts has been well studied in the Panamanian rain forests. To further understand the ecological context of these evolutionary relationships, we have examined the population-genetic structure of the fungus-growing ant species Mycetomoellerius mikromelanos Cardenas, Schultz, Adams 2021 in the Panama Canal Zone. We specifically investigated the presence of population structure, the significance of geographic features (i.e., creeks) limiting gene flow, and relatedness between ant colonies. To accomplish this, we genotyped 85 ant colonies from nine creeks across an approximately 30 km transect in Parque National Soberanía, Panama, using double-digest restriction-site-associated DNA sequencing. We did not find distinct population structure using two genetic clustering methods; however, we did detect an effect of isolation by distance. Furthermore, related colonies were frequently detected on the same creek or neighboring creeks, and some at further geographic distances. Collectively, these findings demonstrate that new colonies tend to establish on natal creeks and occasionally on distant creeks following long-distance dispersal events. We discuss how population-genetic patterns reveal the natural history of M. mikromelanos in Parque National Soberanía and how these results fit into the context of fungus-growing ant mutualisms. Abstract in Spanish is available with online material. 相似文献
5.
V. M. Chavez R. E. Litz P. A. Moon K. Norstog 《In vitro cellular & developmental biology. Plant》1992,28(2):59-63
Summary Embryogenic callus was induced from explanted pinnae of newly emerged leaves of mature plants ofCeratozamia mexicana var. Robusta (Gymnospermae, Cycadales) on a modified B5 formulation with 1 mg·liter−1 kinetin and 1 mg·liter−1 2,4-dichlorophenoxyacetic acid. Proembryos developed on induction medium, but they were more numerous after subculture onto
phytohormone-free medium, which also enabled suspensors to elongate. For nearly 1.5 yr after explanting, subsequent development
of somatic embryos was not observed as suspensors dedifferentiated to form embryogenic callus on phytohormone-free medium.
After this time, cotyledonary somatic embryos developed at the distal end of the suspensors. Somatic embryos have germinated
on phytohormone-free medium. This is the first report of regeneration by somatic embryogenesis of a gymnosperm species from
a mature tree. This technique has great potential for preservation of the highly endangered cycads. 相似文献
6.
R W Larsen M D Chavez D J Nunez M W Davidson D B Knaff T A Krulwich M R Ondrias 《Archives of biochemistry and biophysics》1990,283(2):266-270
The environment of the heme site of a low-potential soluble cytochrome (c552) from alkaliphilic Bacillus firmus RAB has been characterized with resonance Raman scattering and compared to that of horse heart cytochrome c. The Raman data indicate that vibrational bands sensitive to the axial ligation of the heme, as well as modes sensitive to the heme peripheral environment in cytochrome c552, are distinct from those of horse heart cytochrome c. The spectra of cytochrome c552 display resonance Raman modes indicative of a methionine as the sixth ligand in the oxidized form, while the reduced form appears to contain a nitrogenous-based sixth ligand. In addition, Q-band excitation reveals differences among vibrational modes in cytochrome c552 that are sensitive to the amino acid environment surrounding the heme. 相似文献
7.
R W Larsen M D Chavez M R Ondrias S H Courtney J M Friedman M J Lin R E Hirsch 《The Journal of biological chemistry》1990,265(8):4449-4454
Resonance Raman spectroscopy, transient absorption, and fluroescence techniques have been employed to investigate the structure and dynamics of the alpha-cross-linked hemoglobin derivative, HbXL99 alpha. The resonance Raman spectra of the deoxy form of HbXL99 alpha are identical to those of native NbA (VFe-His approximately 222 cm-1), which exhibit a T-state (low affinity) structure regardless of solvent conditions. The resonance Raman spectra of the transient heme photoproduct resulting from CO photolysis from HbXL99 alpha appear to have structures intermediate between deoxy-T and ligand-bound R structures (VFe-His approximately 222 cm-1). Time-resolved resonance Raman data of HbXL99 alpha-CO show that complete CO recombination occurs after approximately 5 ms, with only a small amount of the CO-bound species reforming within approximately 200 ns (geminate recombination). Transient absorption spectra of HbXL99 alpha-O2 indicate that the extent of sub-nanosecond geminate recombination of O2 is also reduced in the cross-linked derivative relative to native HbA. The decrease in tryptophan fluorescence of HbXL99 alpha upon oxygenation further indicates that tertiary structural changes at the alpha 1-beta 2 interface upon ligation are apparently reduced, but not eliminated in the cross-linked derivative relative to HbA. 相似文献
8.
Using a battery of seven lectin-ferritin conjugates as probes for cell surface glycoconjugates, we have studied the pattern of plasmalemmal differentiation of cells in the embryonic rat pancreas from day 15 in utero to the early postpartum stage. Our results indicate that differentiation of plasmalemmal glycoconjugates on acinar, endocrine, and centroacinar cells is temporally correlated with development and is unique for each cell type, as indicated by lectin-ferritin binding. Specifically, (a) expression of adult cell surface saccharide phenotype can be detected on presumptive acinar cells as early as 15 d in utero, as indicated by soybean agglutinin binding, and precedes development of intracellular organelles characteristic of mature acinar cells; (b) maturation of the plasmalemma of acinar cells is reached after intracellular cytodifferentiation is completed, as indicated by appearance of Con A and fucoselectin binding sites only at day 19 of development; conversely, maturation of the endocrine cell plasmalemma is accompanied by "loss" (masking) of ricinus communis II agglutinin receptors; and (c) binding sites for fucose lectins and for soybean agglutinin are absent on endocrine and centroacinar cells at all stages examined. We conclude that acinar, centroacinar, and endocrine cells develop from a common progenitor cell(s) whose plasmalemmal carbohydrate composition resembles most closely that of the adult centroacinar cell. Finally, appearance of acinar lumina beginning at approximately 17 d in utero is accompanied by differenetiation of apical and basolateral plasmalemmal domains of epithelial cells, as indicated by enhanced binding of several lectin-ferritin conjugates to the apical plasmalemmal, a pattern that persists from this stage through adult life. 相似文献
9.
Christian Kiffner John Kioko Jack Baylis Camille Beckwith Craig Brunner Christine Burns Vasco Chavez‐Molina Sara Cotton Laura Glazik Ellen Loftis Megan Moran Caitlin O'Neill Ole Theisinger Bernard Kissui 《Ecology and evolution》2020,10(18):10000-10016
Facilitating coexistence between people and wildlife is a major conservation challenge in East Africa. Some conservation models aim to balance the needs of people and wildlife, but the effectiveness of these models is rarely assessed. Using a case‐study approach, we assessed the ecological performance of a pastoral area in northern Tanzania (Manyara Ranch) and established a long‐term wildlife population monitoring program (carried out intermittently from 2003 to 2008 and regularly from 2011 to 2019) embedded in a distance sampling framework. By comparing density estimates of the road transect‐based long‐term monitoring to estimates derived from systematically distributed transects, we found that the bias associated with nonrandom placement of transects was nonsignificant. Overall, cattle and sheep and goat reached the greatest densities and several wildlife species occurred at densities similar (zebra, wildebeest, waterbuck, Kirk's dik‐dik) or possibly even greater (giraffe, eland, lesser kudu, Grant's gazelle, Thomson's gazelle) than in adjacent national parks in the same ecosystem. Generalized linear mixed models suggested that most wildlife species (8 out of 14) reached greatest densities during the dry season, that wildlife population densities either remained constant or increased over the 17‐year period, and that herbivorous livestock species remained constant, while domestic dog population decreased over time. Cross‐species correlations did not provide evidence for interference competition between grazing or mixed livestock species and wildlife species but indicate possible negative relationships between domestic dog and warthog populations. Overall, wildlife and livestock populations in Manyara Ranch appear to coexist over the 17‐year span. Most likely, this is facilitated by existing connectivity to adjacent protected areas, effective anti‐poaching efforts, spatio‐temporal grazing restrictions, favorable environmental conditions of the ranch, and spatial heterogeneity of surface water and habitats. This long‐term case study illustrates the potential of rangelands to simultaneously support wildlife conservation and human livelihood goals if livestock grazing is restricted in space, time, and numbers. 相似文献
10.
Alain Gagnon Matthew S. Miller Stacey A. Hallman Robert Bourbeau D. Ann Herring David JD. Earn Joaquín Madrenas 《PloS one》2013,8(8)
The worldwide spread of a novel influenza A (H1N1) virus in 2009 showed that influenza remains a significant health threat, even for individuals in the prime of life. This paper focuses on the unusually high young adult mortality observed during the Spanish flu pandemic of 1918. Using historical records from Canada and the U.S., we report a peak of mortality at the exact age of 28 during the pandemic and argue that this increased mortality resulted from an early life exposure to influenza during the previous Russian flu pandemic of 1889–90. We posit that in specific instances, development of immunological memory to an influenza virus strain in early life may lead to a dysregulated immune response to antigenically novel strains encountered in later life, thereby increasing the risk of death. Exposure during critical periods of development could also create holes in the T cell repertoire and impair fetal maturation in general, thereby increasing mortality from infectious diseases later in life. Knowledge of the age-pattern of susceptibility to mortality from influenza could improve crisis management during future influenza pandemics.
“The war is over – and I must go” Egon Schiele, 1890–1918.相似文献