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First identified in 1986 as the catalytic active element in the replication cycle of certain viruses, the hammerhead ribozymes (HHRz) are the smallest known, naturally occurring RNA endonucleases They consist of a single RNA motif which catalyzes a reversible, site-specific cleavage of one of its own phosphodiester bonds . Truncation of this motif allowed a minimal HHRz to be constructed which was the very first ribozyme to be crystallized. HHRz minimal motifs are characterized by a core of eleven conserved nucleotides (bold font in Figure 20) from which three helices of variable length radiate. Selective mutation of any of these conserved residues results in a substantial loss of activity. In the absence of metal ions the structure is relaxed ( extended ), but upon addition of Mg +, hammerhead ribozymes spontaneously fold into a Y-shaped conformation (Figure 20 Color Plate 3). ... 

Fig. 5 Optimized structure of 3n, the selected chemical shifts (regular font for experimental values, and italic for calculated results), and bond lengths (in angstroms). Both the geometry optimization and the NMR studies were calculated at the B3LYP/6-31 l-l-G /Lanl2DZ level...

The RN is the CAS Registry Number and is included whenever the substance is in the CAS Registry File. About half of Du Font s chemicals overlap structures in CAS Registry. The presence or absence of a CAS Registry Number for a Du Pont substance lets the scientist immediately know whether the compound has appeared in the public literature or any publicly available files on STN. The RN field can be used to sort an answer set for compounds that have a CAS registry number. The selected numbers can then be used to find references in STN files that contain CAS Registry Numbers, such as CA File. 

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