GTases are members of the RNA/DNA nucleotidyltransferase superfamily that share six conserved sequence motifs (Fig. 6A) and a similar three-dimensional architecture consisting of an Nterminal NT domain and a C-terminal OB fold domain [21] . As observed previously, the NT domain (aa 1-243) of the C. albicans GTase aligns to the Chlorella virus GTase enzyme with 1.9 Å rmsd over 210 amino acids (26% side chain identity) [53] . In addition, the C. albicans GTase OB domain (aa 244-390) aligns to the Chlorella virus GTase OB domain with 1.8 Å rmsd over 72 amino acids (28% identity) [53] . Because of the high level of structural conservation between members of this family (Fig. 6B) , we hypothesized that MPA should inhibit the GTases of various organisms. We therefore expressed and purified the RNA guanylyltransferases from vaccinia virus and Chlorella virus, as well as the corresponding human enzyme. Our results demonstrate that MPA inhibits all these GTases to the same extent than the S. cerevisiae homolog (Fig. 6C) . IC 50 values similar to the ones obtained for both the first (,3 mM) and second step (,620 mM) of the GTase reaction were observed when MPA was added to the reactions catalyzed by the vaccinia, Chlorella, and human enzymes. Overall, these results indicate that MPA inhibits the activity of different GTases through a similar mechanism that mainly prevents the catalytic transfer of the GMP moiety onto an acceptor RNA.