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pubmed:abstractText |
Large tumor (T) antigen and its bound multimeric states are positioned by scanning transmission electron microscopy (STEM) within a few base pairs at control sequences of the simian virus 40 DNA origin of replication region. Proximal and distal edge positions for each multimer group match the end positions of previously mapped fragments protected from DNase cleavage. Since chance correspondence is shown to be extremely unlikely, STEM mass measurements, obtained concurrently with STEM map positions, indicate that the DNase fragments arise from bound monomers, dimers, trimers, and tetramers in binding region II and monomers, dimers, and trimers in binding region I. Simultaneous binding of seven monomer-equivalent masses is observed, three in region I and four in region II, with an ordered and interpretable mass distribution in the plane of the foil. Although this observation does not prove that the six G-A-G-G-C and one T-A-G-G-C sequences, similarly distributed, function as recognition sequences for T-antigen monomer, it provides strong support for such a model. The stable existence in solution of low-and intermediate-mass structures, observed at lower T-antigen concentrations, suggests a role as assembly intermediates.
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