FIGURE 4.

RNA ligands (A) rU₆ and (B) rC7 surrounded by their electron density. The ssRNA is colored according to the CPK scheme with the exception of carbon, which is green. Their composite omit electron densities are contoured corresponding to 1.0 σ. (A) Five of six uridine nucleotides of rU₆ could be built into the electron density. The phosphodiester group connecting nucleotides 2 and 3 adopts a double conformation. (B) Nucleotides U2 to U6 of rC7 could be built into the density. Nucleotide C3 and the phosphodiester group of the following nucleotide exist in two conformations. The occupancy of U2 is 0.5 such that a sterical clash with the C3 nucleotide which would collide in one of its two conformations is avoided.