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D M Silver 《Cellular immunology》1979,48(2):249-257
The role of genes linked to the H-2 locus in effecting an immune response to SRBC was examined using strains of mice which differ in the classes of antibodies produced following multiple injections with SRBC. While H-2-linked gene action appeared to be at the level of regulating the number of plaque-forming cells (PFC) present in the spleens of different strains following two injections with SRBC, non-H-2-linked immune response genes seemed to determine whether an IgM-IgG switch occurred as well as how much of each antibody class was produced by the number of PFC available as a result of H-2-linked gene intervention. Mapping studies showed that the H-2-linked genetic effects were due to either the requirement for two genes or the presence of genes located between I-B and H-2D. 相似文献
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Evolution of asymbiotic nitrogen fixation 总被引:3,自引:0,他引:3
Recent observations on the nature of the enzyme complex, nitrogenase, prepared from a variety of nitrogen-fixing micro-organisms, on its substrate specificity, energy requirements, source of reducing power and sensitivity to O2 now permit speculation on the evolution of biological nitrogen fixation in asymbiotic micro-organisms.Ability to fix N2 is restricted to procaryotic organisms and is particularly widespread among those having characteristics (e.g. hydrogenase, ferredoxin) regarded as primitive. If the primitive environment was devoid of O2, the earliest N2-fixing prokaryote would have been a strict anaerobe, not unlike Clostridium pasteurianum. Yet N2-fixation seems unnecessary in a primitive ammonia-containing environment, and ammonia represses this function in contemporary species. This apparent paradox, the development of the ability to fix N2 in circumstances in which it was apparently unnecessary suggests that a substance other than N2 might have been primary substrate of the primeval enzyme.Substances such as acetylene, cyanide, cyanogen, nitriles or isonitriles are all substrates for nitrogenase and are all probable components of the primitive terrestrial environment. Biologically useful functions which a nitrogenase-like reductase system might have served involving substrates other than N2 include: (a) a detoxification reaction to nullify the effects of cyanide or cyanogen; (b) a means of generating ATP anaerobically; (c) a hydrogen “escape valve”.Functions (b) and (c) are improbable because they would be physiologically uneconomic; function (a) is plausible.With the emergence of an oxidizing atmosphere, facultative and aerobic N2-fixing micro-organisms could only retain the nitrogenase system if the O2-sensitive component was protected from inactivation. In the Azotobacteraceae this is achieved by “conformational protection” together with a high respiration rate; in blue-green algae, a structural compartmentation occurs in the more highly evolved species. 相似文献
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A. L. Leigh Silver 《BMJ (Clinical research ed.)》1936,2(3964):1333-1334
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