Differences in the phosphate oxygen requirements for self-cleavage by the extended and prototypical hammerhead forms.

The hammerhead self-cleaving motif occurs in a variety of RNAs that infect plants and consists of three non-conserved helices connected by a highly conserved central core. A variant hammerhead, called the extended hammerhead, is found in satellite 2 transcripts from a variety of caudate amphibians. The extended hammerhead has the same core as the prototypical hammerhead, but has unusually conserved sequence and structural elements in its peripheral helices. Here we present the results of a thiophosphate substitution interference analysis of the pro-Rp phosphate oxygen requirements in the two hammerhead forms. Five pro-Rp phosphate oxygens, all in the central core, were found to be important for self-cleavage by the prototypical hammerhead. A similar set of core positions were important for self-cleavage by the extended hammerhead, but five non-core positions were also found to be important. Thiosubstitution at one of these positions had the most severe effect on self-cleavage observed in this analysis. Mn2+ did not alleviate this negative effect, indicating that this position was not part of a divalent cation binding site. We propose that novel tertiary interactions in the extended hammerhead help form the same catalytic core structure as that used by the prototypical plant virus hammerhead.