Nuclear Magnetic Resonance Spectroscopy and Molecular Modeling Reveal That Different Hydrogen Bonding Patterns Are Possible for G·U Pairs: One Hydrogen Bond for Each G·U Pair in r(GGCGU<
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- 作者：Xiaoying Chen ; Jeffrey A. McDowell ; Ryszard Kierzek ; Thomas R. Krugh ; and Douglas H. Turner
- 刊名：Biochemistry
- 出版年：2000
- 出版时间：August 1, 2000
- 年：2000
- 卷：39
- 期：30
- 页码：8970 - 8982
- 全文大小：193K
- 年卷期：v.39,no.30(August 1, 2000)
- ISSN：1520-4995
文摘
G·U pairs occur frequently and have many important biological functions. The stability ofsymmetric tandem G·U motifs depends both on the adjacent Watson-Crick base pairs, e.g., 5'G > 5'C,and the sequence of the G·U pairs, i.e., 5'-UG-3' > 5'-GU-3', where an underline represents a nucleotidein a G·U pair [Wu, M., McDowell, J. A., and Turner, D. H. (1995) Biochemistry 34, 3204-3211]. Inparticular, at 37 
C, the motif 5'-CGUG-3' is less stable by approximately 3 kcal/mol compared withother symmetric tandem G·U motifs with G-C as adjacent pairs: 5'-GGUC-3', 5'-GUGC-3', and 5'-CUGG-3'. The solution structures of r(GAGUGCUC)2 and r(GGCGUGCC)2 duplexes have been determinedby NMR and restrained simulated annealing. The global geometry of both duplexes is close to A-form,with some distortions localized in the tandem G·U pair region. The striking discovery is that inr(GGCGUGCC)2 each G·U pair apparently has only one hydrogen bond instead of the two expected fora canonical wobble pair. In the one-hydrogen-bond model, the distance between GO6 and UH3 is too farto form a hydrogen bond. In addition, the temperature dependence of the imino proton resonances is alsoconsistent with the different number of hydrogen bonds in the G·U pair. To test the NMR models, U orG in various G·U pairs were individually replaced by N3-methyluridine or isoguanosine, respectively,thus eliminating the possibility of hydrogen bonding between GO6 and UH3. The results of thermal meltingstudies on duplexes with these substitutions support the NMR models.
C, the motif 5'-CGUG-3' is less stable by approximately 3 kcal/mol compared withother symmetric tandem G·U motifs with G-C as adjacent pairs: 5'-GGUC-3', 5'-GUGC-3', and 5'-CUGG-3'. The solution structures of r(GAGUGCUC)2 and r(GGCGUGCC)2 duplexes have been determinedby NMR and restrained simulated annealing. The global geometry of both duplexes is close to A-form,with some distortions localized in the tandem G·U pair region. The striking discovery is that inr(GGCGUGCC)2 each G·U pair apparently has only one hydrogen bond instead of the two expected fora canonical wobble pair. In the one-hydrogen-bond model, the distance between GO6 and UH3 is too farto form a hydrogen bond. In addition, the temperature dependence of the imino proton resonances is alsoconsistent with the different number of hydrogen bonds in the G·U pair. To test the NMR models, U orG in various G·U pairs were individually replaced by N3-methyluridine or isoguanosine, respectively,thus eliminating the possibility of hydrogen bonding between GO6 and UH3. The results of thermal meltingstudies on duplexes with these substitutions support the NMR models.