The hairpin ribozyme is a short endonucleolytic RNA motif
isolated from a family of related plant virus satellite
RNAs. It consists of two independently folding domains,
each comprising two Watson–Crick helices flanking
a conserved internal loop. The domains need to physically
interact (dock) for catalysis of site-specific cleavage
and ligation reactions. Using tapping-mode atomic force
microscopy in aqueous buffer solution, we were able to
produce high quality images of individual hairpin ribozyme
molecules with extended terminal helices. Three RNA constructs
with either the essential cleavage site guanosine or a
detrimental adenosine substitution and with or without
a 6-nt insertion to confer flexibility to the interdomain
hinge show structural differences that correlate with their
ability to form the active docked conformation. The observed
contour lengths and shapes are consistent with previous
bulk-solution measurements of the transient electric dichroism
decays for the same RNA constructs. The active docked construct
appears as an asymmetrically docked conformation that might
be an indication of a more complicated docking event than
a simple collapse around the interdomain hinge.