Three-dimensional model of the active site of the self-splicing rRNA precursor of Tetrahymena.

The rRNA intervening sequence of Tetrahymena is a catalytic RNA molecule, or "ribozyme." A tertiary-structure model of the active site of this ribozyme has been constructed based on comparative sequence analysis of related group I intervening sequences, data on the accessibility of each nucleotide to chemical and enzymatic probes, and principles of RNA folding derived from a consideration of the structure of tRNA determined by x-ray crystallography. In the model, the catalytic center has a two-helix structural framework composed of the base-paired segments of the group I conserved sequence elements. The structural framework supports and orients the conserved nucleotides that are adjacent to the base-paired sequence elements; these conserved nucleotides are proposed to form the active site and to bind the 5' splice-site duplex and the guanine nucleotide substrate. Tests of the model are proposed.