Fig. 1.

Overall structure of the SRP54–SRP19–7S.S RNA complex. (A) The nucleotide sequence and two-dimensional topology diagram of the 7S.S RNA fragment used for crystallization are given with the numbering corresponding to full-length M. jannaschii 7S RNA. The lines between bases indicate Watson–Crick base pairs, the open circles indicate noncanonical pairs, and the filled circles indicate tertiary RNA–RNA interactions. Highlighted in green are the symmetric and asymmetric loops in helix 8. (B) Ribbon representation of the asymmetric unit of the crystal, which contains two S domains. Their respective RNAs are stacked end to end, forming a pseudocontinuous helical element. In S domains A and B, the NG domain is shown in blue and light blue, and 7S RNA is shown in red and light red, respectively; the M domain is shown in green; the GM-linker is shown in orange; and SRP19 is shown in yellow. The 25-residue finger loop (indicated by dots) in the M domains was not observed in the electron density maps. (C) Top and side view of the structure of S domain A. The molecular surface of the NG domain highlights the good fit with 7S.S RNA. The noncrystallographic twofold axis is indicated by an arrow. The images are color-coded as in A.