Components of UGGGGU octamer assembly. (a) An isolated RNA hexamer, the 3′ uridine that participates in a U tetrad, is shown in blue, and the 5′ uridine involved in tetramer intercalation is shown in cyan. Two central Sr2+ cations involved in G-quartet stabilization are shown in magenta. (b) The tetramer component produced by rotation of an individual strand about a 4-fold axis. (c) Two adjacent, stacked, strands related by 2-fold symmetry and (d) the resulting octamer. Coordinates correspond to data set P1B.