Scheme 1.

SV40 sequence from 1905 to 1965 bp, with the location of the apparent bend center denoted by a vertical arrow. The individual A- and T-tracts in the curvature module are designated in bold by the compact notation, A₆, A₄T, A₄, A₃T₄ and T₇, that will be used in the subsequent text. Note that the second A-tract is designated A₄T, both to distinguish it from the following A₄-tract and in recognition of the fact that thymine residues following an A-tract exhibit many of the properties of the preceding adenine residues (16–19).