The fine structure of the melting curve for the linear ColE1 DNA has been obtained. To find the ColE1 DNA regions corresponding to peaks in the melting curve's fine structure, we fixed the melted DNA regions with glyoxal /12/. Electron-microscopic denaturation maps were obtained for nine temperature points within the melting range. Thereby the whole process of ColE1 DNA melting was reconstructed in detail. Spectrophotometric and electron microscopic data were used for mapping the distribution of GC-pairs over the DNA. molecule. The most AT-rich DNA regions (28 and 37% of GC-pairs), 380 and 660 bp long resp., are located on both sides of the site of ColE1 DNA's cleavage by EcoR1 endonuclease. The equilibrium denaturation maps are compared with maps obtained by the method of Inman /20/ for eight points of the kinetic curve of ColE1 DNA unwinding by formaldehyde.
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