Maps calculated at 1.8 Å resolution
with SAD phases for the Rb-FET structure and superimposed on the
final structure (selected DNA residues are numbered). (A)
The anomalous difference Fourier (blue) and double-difference (red)
maps contoured at the 4σ level. Ion-binding
sites 1 and 2 (shown with their symmetry mates, designated by lower
case letters) were located in the anomalous difference Patterson
map and used for phasing. Ion-binding site 3 was found at this stage
and is characterized by a significantly greater peak height than
the remaining peaks in the double-difference density map. (B)
Stereo diagram of the density-modified electron density map contoured
at the 1σ level and superimposed on
the central TAT portion of the structure. DNA atoms are colored
yellow, red, cyan, orange and green for carbon, oxygen, nitrogen,
phosphorus and fluorine, respectively. Electron density for the
FET substituent of T6 is clearly visible in this map.