FIGURE 8.

Model of SRP assembly in M. jannaschii. Binding of SRP19 to 7S RNA causes an induced fit of the helix 6 tetraloop and a repositioning of the distal helix 6 relative to helix 8. The altered positions of A176 and A177 stabilize an asymmetric loop structure with the bases A195–C197 surface exposed. These bases become subsequently stacked upon interaction with SRP54M. Thus, the stabilization of a bulge structure, which resembles the one in the SRP54-bound form, accounts for the enhanced affinity of SRP54 for 7S RNA in the presence of SRP19. The positions of C198 (*) and the G193:A194 base step (#) situated in the long strand of the asymmetric loop are indicated. Important tertiary RNA contacts are indicated in green. The RNA helical segments and nucleotides are colored as in Figure 2.