BIDIRECTIONAL EFFECTORS OF A GROUP-I INTRON RIBOZYME

The group I self-splicing introns found in many organisms are competitively inhibited by L-arginine. We have found that L-arginine acts stereoselectively on the Pc1.LSU nuclear group I intron of Pneumocystis carinii, competitively inhibiting the first (cleavage) step of the splicing reaction and stimulating the second (ligation) step. Stimulation of the second step is most clearly demonstrated in reactions whose first step is blocked after 15 min by addition of pentamidine. The guanidine moiety of arginine is required for both effects, L-Canavanine is a more potent inhibitor than L-arginine yet it fails to stimulate. L-Arginine derivatized on its carboxyl group as an amide, ester or peptide is more potent than L-arginine as a stimulator and inhibitor, with di-arginine amide and tri-arginine being the most potent effecters tested. The most potent peptides tested are 10 000 times as effective as L-arginine in inhibiting ribozyme activity, and nearly 400 times as effective as stimulators, Arginine and some of its derivatives apparently bind to site(s) on the ribozyme to alter its conformation to one more active in the second step of splicing while competing with guanosine substrate in the first step. This phenomenon indicates that ribozymes, like protein enzymes, can be inhibited or stimulated by non-substrate low molecular weight compounds, which suggests that such compounds may be developed as pharmacological agents acting on RNA targets.