Self-cleavage sites in the RNA (ribozymes) of hepatitis delta virus (HDV) were hypothesized, identified, and initially characterized independently by Taylor and coworkers (Kuo et al. 1988; Sharmeen et al. 1988) and Wu et al. (1989). The ribozymes were proposed to process primary replication products, generated in rolling circle replication, to monomer size (Robertson 1992; MacNaughton et al. 1993; Lazinski and Taylor 1995a). Specific subfragments of each RNA strand were found to contain both the cleavage site and the sequences necessary and sufficient for self-cleavage (Kuo et al. 1988; Wu et al. 1989). Although thesequences associated with thecleavagesites in HDVRNA differedfromthe sequence that formed the familiar hammerhead and hairpin ribozyme motifs, the reaction was the same. Self-cleavage of the RNA backbone involves a rearrangement of the 3',5' phosphodiester bond to generate a 2',3'-cyclic phosphate group and a 5' hydroxyl group, suggesting nucleophilic attack of the adjacent 2' hydroxyl on the scissile phosphate (Wu et al. 1989). The in vitro reaction required no protein or cellular factor but a divalent metal ion greatly stimulated cleavage rates (Wu et al. 1989). This chapter will review biochemical and structural data with emphasis on experiments and results that help us understand the catalytic mechanism used by the HDV rib ozymes.

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