The ability of MPA to inhibit the GTase of the model organism S. cerevisiae (Ceg1 protein) was then investigated. The GTase activity is actually a two-step ping-pong reaction in which the enzyme first reacts with GTP to produce the enzyme-GMP (EpG) covalent intermediate with the concomitant release of pyrophosphate [2] . In the second step of the reaction, the GMP moiety is then transferred to a 59-diphosphate RNA. The ability of MPA to inhibit the complete GTase reaction (both steps) was monitored using a standard GTase assay in which the purified enzyme was incubated with [alpha-32 P]GTP and a 59-diphosphate acceptor RNA. The reaction products were then analyzed on a denaturing polyacrylamide gel. As can be seen in figure 2B , the presence of increasing concentrations of MPA significanlty decreased the transfer of the radiolabeled GMP moiety onto the acceptor RNA. MPA was shown to inhibit the overall GTase activity by 50% at 360 mM, and by 80% at 900 mM.