FIGURE 6-33 C:U base pair. The structure shews hydrogen bonds that allow base pfiiHng to occur between guanine and uracil.

A feature of RNA that adds to its propensity to form double-helical structures is an additional, non-Watson-Crick base pair. This is the G:tJ base pair, which has hydrogen bonds between N3 of uracil and the car-bony! on CfS of guanine and between the oarbonyl on C2 of uracil and N1 of guanine (Figure 6-33}. Because G;U base pairs nan occur as well as the (our conventional, Watson-Crick base pairs, RNA chains have an enhanced capacity for self-complementarity. Thus, RNA frequently exhibits local regions of base pairing but not the long-range, regular helicity of DNA,

The presence of 2'-hydroxyIs in the RNA backbone prevents RNA from adopting a B-form helix. Rather, double-helical RNA resembles the A-form structure of UNA. As such, the minor groove is wide and shallow, and hence accessible, but recall that the minor groove offers little sequence-specific information. Meanwhile, the major groove is so narrow and deep that it is not very accessible to amino acid side chains from interacting proteins. Thus, the RNA double helix is quite distinct from the DNA double helix in its detailed atomic structure and less well suited for sequence-specific interactions with proteins (although some proteins do bind to RNA in a sequence-specific manner),

RNA Can Fold Up into Complex Tertiary Structures

Freed of the constraint of forming long-range regular helices, RNA can adopt a wealth of tertiary structures. This is because RNA has enormous rotational freedom in the backbone of its non-base-paired regions. Thus, RNA can fold up into complex tertiary structures frequently involving unconventional base pairing, such as the base triples and base-backbone interactions seen in tRNAs (see, for example, the illustration of the IJ:A:l.I base triple in Figure 6-34). Proteins can assist the formation of tertiary structures by large RNA molecules, such as those found in the ribosome. Proteins shield the negative charges of backbone phosphates, whose electrostatic repulsive forces would otherwise destabilize the Structure.

Researchers have taken advantage of the potential structural complexity of RNA to generate novel RNA species (no1 found in nature) that

FIGURE 6-J4 U:fl:U base triple. The structure shows one example ot hydrogen bonding that allows unusual triple base pairing.

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