(A) Open-box view of 5' ETS and U3 RNA. The left view shows an intact 90S structure. The right view is resulted from removal of the 5' and 3' domain of small subunit, Utp30, Enp1, Unk2, Utp20, Bms1, Rcl1, Enp2 and Bfr2. The unassigned structures are colored grey. (B) A 90° rotation of A. UTPB is additionally removed. (C) Structure of 5' ETS and the 5' domain of U3 RNA. Same orientation as in B. (D) The ΔMtr4 EM density for 5' ETS and the 5' domain of U3 RNA. (E) H1. (F) H2 and H3. A view of the mouth. (G) H4 and H7. (H) Ha and Hb. (I) H6, H8 and H9. (J) Imp3 binds at the junction of H6, H7, Ha and Hb. Two perpendicular views are shown. (K) Hc and Hd. The 5' nucleotides G7 and G1111 of the 18S strands in Hc and Hd are labeled.