Fig. 2.

Unusual features of the structure. (A) A hydrogen bond is able to form between C₃₃N4 and the phosphate of U₃₆. ASL carbons are colored green, and mRNA carbons are blue (m2F_O-dF_C contoured at 1.5 σ). (B) (green) is superimposed with E. coli elongator (blue) and human elongator (black), showing a slightly distorted conformation. (C) In E. coli , the cross-loop interaction involves the familiar C₃₂•A₃₈ noncanonical base pair with A₃₇ displaced from the loop, where it cannot participate in base stacking. (D) A cross-loop interaction in involves a unique interaction between C₃₂ and A₃₇ that consists of a base pair between the Watson–Crick edge of C₃₂ and the Hoogsteen edge of A₃₇.