Nucleic Acids in Chemistry and Biology

256 Chapter 7

N

N N

N

N

C 1 '

N

N

N

N

C 1 '

O

N

N

H

N

N

N

O NH 2

C 1 '

a

N

N N

N

O

N

C 1 '

N N

C 1 '

N

O

N

N

C 1 ' N N

O

N

b

H H

H

H

H

H

H

H

H H

H

H

H

Figure 7.4 Examples of the unusual triple interactions observed in RNA. (a) One type of GGA triplex (GGA N-7-imino,
carbonyl-amino; N-3-amino, amino-N-7) observed in tRNA bound to a synthetase.^4 (b) A type of GGC
triplex (GGC amino(N-2)-N-7, imino-carbonyl, carbonyl-amino(N-4); Watson–Crick) observed in the
50S ribosome^5

7.1.3 RNA Multiple Interactions

In addition to base pairing, RNA nucleotides commonly participate in multiple interactions that involve
both the bases and the sugars. These interactions are sometimes observed individually within a tertiary struc-
ture,^3 or they can be linked together to form extended triplexes and quadruplexes (see also Sections 2.4.5 and
2.3.7, respectively). Multiplexes such as the square guanosine tetraplex (G-quartet, G-tetrad)are similar in both
RNA and DNA (Section 2.3.7). However, a greater diversity of triple and quadruple interactions is observed
in RNA molecules. For example, an all-purine triple interaction that is dominated by Hoogsteen contacts
is observed in the complex of a seryl tRNA synthetase with its cognate tRNA (Figure 7.4a).^4 The 50S

N

N

N

N

N

N

O

NH 2

C 1 '

O

O

H

H

a

N

N N

N N

N

N NH

N

C 1 '

N

c O

H

H

H

H

N

N

N

N

N

N

N

C 1 '

H 2 N

O

b

H

H

H

Figure 7.3 Common forms of alternative base pairing in RNA. The pink line indicates the base plane axis for a wobble
pairing. (a) G–U wobble pair. (b) A–C pair. (c) Sheared form of the G–A pair

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