FIGURE 4.

Yeast Thg1 catalyzes template-dependent 3′-to-5′ polymerase activity. Utilizing tRNA^His variant substrates that contain C₇₃ instead of the wild-type A₇₃, yeast Thg1 catalyzes sequential addition of up to three G-residues to the 5′ end of the tRNA. The reverse polymerase reaction only occurs in the presence of the correct Watson-Crick base-pairing nucleotide (GTP in the case of the C₇₃-tRNA). For multiple nucleotide additions, the 5′-triphosphorylated end resulting from the previous nucleotide addition is the activated end for attack by the 3′-hydroxyl of the subsequent nucleotide, as shown in detail in brackets.