UV-crosslinking between the internally 32 P-labeled RNA of 81 nt and the yeast GTase was performed in a crosslink buffer (50 mM Tris pH 7.5, 5 mM DTT, 5 mM MgCl 2 ). The protein (12 mM) was incubated with GTP (1 mM) and different concentrations of MPA for 10 minutes at 30uC. The radiolabeled RNA (3 mM) was added to the reaction mixture and incubated for 5 minutes at 30uC. The reaction mixture was exposed to UV light (254 nm, 20 Joules/cm 2 ) for 5 min at 30uC using a Stratalinker 2400 UV Crosslinker (Stratagene). The crosslink mixture was denatured (50 mM Tris-HCl, pH 7.0, 5% sucrose, 5% bmercaptoethanol, 2% SDS) and separated by electrophoresis on a 12% SDS-PAGE. The gel was analyzed by phosphorimaging.