through the collar by using substructures that engage the DNA bases, much like a gear that engages inden-
tations in a conveyor belt.
7.6.2 RNA as a Catalyst: Self-Cleaving Motifs from Viral RNA
Although the phenomenon of RNA catalysis was discovered initially during studies of eukaryotic
RNAs, such as the Tetrahymenagroup I intron and RNAse P RNA, other importantribozymes, such as
hammerhead, hairpin, and hepatitis delta were initially derived from self-cleaving RNA motifsthat are
common in plant and animal viruses (Figures 7.37 and 7.9).^68
RNA Structure and Function 285
Figure 7.35 Cryo-EM representation of the phage packaging motor and its RNA components. (a) The intact phage
particle, with arrows indicating the collar region. (b) Cutaway view of the capsid shell with its ring of
RNA at the base (circled in red). (c) A top view of the RNA oligomeric ring
(Reprinted from Ref. 66. © (2000), with permission from Macmillan Publications Ltd)
Figure 7.36 Secondary structure of monomeric pRNA from phi29 bacteriophage
(Adapted from Ref. 79. © (2001), with permission from ASBMB)