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61.
Erwin F. Jenny Marjorie C. Caserio John D. Roberts 《Cellular and molecular life sciences : CMLS》1958,14(10):349-354
Summary Many substitution reactions of aromatic halides, such as amination with metallic amides in ammonia or amine solutions, high-temperature alkaline hydrolyses and arylation with phenyllithium, lead to the formation of rearrangement products as the result of a common reaction path involving unstable benzyne intermediates. The chemical evidence in favor of this reaction mechanism is discussed as are correlations of observed product compositions and applications to practical synthetic procedures.
Contribution No. 2390. 相似文献
Contribution No. 2390. 相似文献
62.
Reciprocity, whether direct or indirect, is thought to be the key to establishing cooperation among non-relatives. But Riolo et al. have presented a model in which cooperation is instead based on similarity: agents donate only when their partner's 'tag' lies within a 'tolerance' range around their own. Here we point out that their model requires individuals with identical tags to cooperate with each other, and show that cooperation tends to collapse when individuals bearing identical tags are given the option of not donating. We therefore question their mechanism for maintaining cooperation without reciprocity. 相似文献
63.
64.
A mouse knock-in model exposes sequential proteolytic pathways that regulate p27Kip1 in G1 and S phase 总被引:18,自引:0,他引:18
Malek NP Sundberg H McGrew S Nakayama K Kyriakides TR Roberts JM Kyriakidis TR 《Nature》2001,413(6853):323-327
The protein p27Kip1 is an inhibitor of cell division. An increase in p27 causes proliferating cells to exit from the cell cycle, and a decrease in p27 is necessary for quiescent cells to resume division. Abnormally low amounts of p27 are associated with pathological states of excessive cell proliferation, especially cancers. In normal and tumour cells, p27 is regulated primarily at the level of translation and protein turnover. Phosphorylation of p27 on threonine 187 (T187) by cyclin-dependent kinase 2 (Cdk2) is thought to initiate the major pathway for p27 proteolysis. To critically test the importance of this pathway in vivo, we replaced the murine p27 gene with one that encoded alanine instead of threonine at position 187 (p27T187A). Here we show that cells expressing p27T187A were unable to downregulate p27 during the S and G2 phases of the cell cycle, but that this had a surprisingly modest effect on cell proliferation both in vitro and in vivo. Our efforts to explain this unexpected result led to the discovery of a second proteolytic pathway for controlling p27, one that is activated by mitogens and degrades p27 exclusively during G1. 相似文献
65.
Human transforming growth factor-beta complementary DNA sequence and expression in normal and transformed cells 总被引:17,自引:0,他引:17
R Derynck J A Jarrett E Y Chen D H Eaton J R Bell R K Assoian A B Roberts M B Sporn D V Goeddel 《Nature》1985,316(6030):701-705
The partial amino-acid sequence of purified human transforming growth factor-beta (TGF-beta) was used to identify a series of cDNA clones encoding the protein. The cDNA sequence indicates that the 112-amino acid monomeric form of the natural TGF-beta homodimer is derived proteolytically from a much longer precursor polypeptide which may be secreted. TGF-beta messenger RNA is synthesized in various normal and transformed cells. 相似文献
66.
This paper reconsiders event study methodology, a very popular technique in the applied finance literature, within the context of testing cumulative prediction errors. It extends the conventional test statistics in two directions. First, it accounts fully for the increased variance of prediction errors outside of the estimation period and for the cumulation of these errors across different event windows. Second, it also takes account of the fact that market model residuals are typically serially correlated, heteroscedastic and non-normal. The statistics are compared with the conventional approach by reassessing a previous application of the methodology to the impact of management buyouts. 相似文献
67.
The antiviral activity of alpha-interferon (IFN-alpha) produced by human leukocytes in response to viral infection has been considered to be independent of the virus which induced its production. Recently, however, IFN-alpha has been found to include at least eight different subtypes, as indicated by measurement of antigenic variability, DNA hybridization and amino acid sequencing. We considered the possibility that interferon heterogeneity may play a part in enhancing host antiviral defence and now present data suggesting that purified human macrophages, when exposed to different viruses, produce interferons having differing spectra of antiviral activity. These findings may provide a functional correlation for IFN-alpha heterogeneity. 相似文献
68.
E. coli recA protein-directed cleavage of phage lambda repressor requires polynucleotide 总被引:70,自引:0,他引:70
The recA protein mediates both genetic recombination and several cellular responses to DNA damage, including the induction of temperate bacteriophage. Indication of phage lambda results from proteolytic cleavage of lambda repressor directed by recA protein. We show here that this cleavage reaction requires both polynucleotide and ATP. We suggest that a stoichiometric complex of recA protein and DNA is active both to destroy repressors by proteolytic cleavage and to initiate pairing of this DNA to its homologous sequence in a DNA duplex ('strand invasion'). 相似文献
69.
Molecular mechanism for missense suppression in E. coli 总被引:4,自引:0,他引:4
70.