Published online by Cambridge University Press: 24 April 2002
In the second step of the two consecutive transesterifications of the self-splicing reaction of the group I intron, the conserved guanosine at the 3′ terminus of the intron (ωG) binds to the guanosine-binding site (GBS) in the intron. In the present study, we designed a 22-nt model RNA (GBS/ωG) including the GBS and ωG from the Tetrahymena group I intron, and determined the solution structure by NMR methods. In this structure, ωG is recognized by the formation of a base triple with the G264[bull ]C311 base pair, and this recognition is stabilized by the stacking interaction between ωG and C262. The bulged structure at A263 causes a large helical twist angle (40 ± 8°) between the G264[bull ]C311 and C262[bull ]G312 base pairs. We named this type of binding pocket with a bulge and a large twist, formed on the major groove, a “Bulge-and-Twist” (BT) pocket. With another twist angle between the C262[bull ]G312 and G413[bull ]C313 base pairs (45 ± 10°), the axis of GBS/ωG is kinked at the GBS region. This kinked axis superimposes well on that of the corresponding region in the structure model built on a 5.0 Å resolution electron density map (Golden et al., Science, 1998, 282:345–358). This compact structure of the GBS is also consistent with previous biochemical studies on group I introns. The BT pockets are also found in the arginine-binding site of the HIV-TAR RNA, and within the 16S rRNA and the 23S rRNA.