HDV replication depends on small self-cleaving sequences in both the ge-nomic and antigenomic strands of the replication intermediates. These ri-bozymes have provided a rich system for studying catalytic RNA and this re-viewhas focused on the chemical and catalytic mechanism of those ribozymes. However, it has ignored the equally interesting questions concerning the need for controlling cleavage/ligation activities and integrating ribozyme activity into the biology of the virus (MacNaughton et al. 1993; Lazinski and Taylor 1995b, 1995a). Ribozymes are typically thought to require metal ions or other cations for structural stability and usually, but not always, thought to use divalent metal ions for a direct catalytic role as well. Large ribozymes such at the self-splicing introns and RNase P RNA are probably correctly classified as obligate metallo-ribozymes. The picture is different for the small self-cleaving ribozymes; the hairpin, hammerhead, VS and HDV ribozyme all show vari ous levels of activity in the absence of divalent metal ions (Murray et al. 1998; Curtis and Bartel 2001; Nakano et al. 2001; O'Rear et al. 2001; Wadkins et al. 2001). The hairpin ribozyme does not require a divalent metal ion (Ham-pel and Cowan 1997; Young et al. 1997) and the others might be considered metal-assisted since they cleave faster when Mg2+ is available. Of these, the HDV ribozymes have provided the most evidence for how the RNA may be directly involved in catalysis. The possibility that RNA nucleobases, like some familiar amino acid side chains, would be capable of acid-base catalysis had been discussed, but in the absence of a good example, it was beginning to look unlikely (Cech and Golden 1998). However, in providing what now appears to be strong evidence for cytosine (and adenine) catalyzed proton transfer, the ribozymes from HDV have made a unique contribution to our understanding of the RNA world hypothesis.

Acknowledgements I thank A. Perrotta, S. Wilkinson, A. Brown and S. Chamberlin for comments on earlier version of the manuscript. I am especially grateful to Drs. S. Das and J. Piccirilli for providing their manuscript prior to publication. I also wish to acknowledge fun discussions of ideas over the years with Drs. J. Doudna, A. Ke, P. Bevilacqua and others working with the HDV ribozymes. This work was supported by a grant from the National Institutes of Health (GM047233).

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