1t4x: Difference between revisions

[[Image:1t4x.gif|left|200px]]

'''The first left-handed RNA structure of (CGCGCG)2, Z-RNA, NMR, 12 structures, determined in high salt'''

Right-handed RNA duplexes of (CG)n sequence undergo salt-induced helicity reversal, forming left-handed RNA double helices (Z-RNA). In contrast to the thoroughly studied Z-DNA, no Z-RNA structure of natural origin is known. Here we report the NMR structure of a half-turn, left-handed RNA helix (CGCGCG)2 determined in 6 M NaClO4. This is the first nucleic acid motif determined at such high salt. Sequential assignments of non-exchangeable proton resonances of the Z-form were based on the hitherto unreported NOE connectivity path [H6(n)-H5'/H5''(n)-H8(n+1)-H1'(n+1)-H6(n+2)] found for left-handed helices. Z-RNA structure shows several conformational features significantly different from Z-DNA. Intra-strand but no inter-strand base stacking was observed for both CpG and GpC steps. Helical twist angles for CpG steps have small positive values (4-7 degrees), whereas GpC steps have large negative values (-61 degrees). In the full-turn model of Z-RNA (12.4 bp per turn), base pairs are much closer to the helix axis than in Z-DNA, thus both the very deep, narrow minor groove with buried cytidine 2'-OH groups, and the major groove are well defined. The 2'-OH group of cytidines plays a crucial role in the Z-RNA structure and its formation; 2'-O-methylation of cytidine, but not of guanosine residues prohibits A to Z helicity reversal.

Full crystallographic information is available from [http://oca.weizmann.ac.il/oca-bin/ocashort?id=1T4X OCA].  

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