IDENTIFICATION OF THE NUCLEOTIDES IN THE A-RICH BULGE OF THE TETRAHYMENA RIBOZYME RESPONSIBLE FOR AN EFFICIENT SELF-SPLICING REACTION

P5abc is a large extension of the PS element characteristic of subclas ses IC1 and IC2 of group I introns. It has a conserved region termed t he A-rich bulge, that is responsible for activation of the Tetrahymena self-splicing intron. By employing a modified color-colony assay syst em, we identified four adenosines in the bulge that are responsible fo r an efficient splicing reaction. On comparison with the,X-ray crystal structure of the P4-5-6 domains of the Tetrahymena intron, three aden osines at positions 183, 184, and 186 were found to be identical to th ose significantly contributing to the formation of its tertiary struct ure, However, our results show that an adenosine at 187 is involved in the formation of a Watson-Crick base pair with U135, although it form s a Hoogsteen base pair in the crystal structure.