Figure 6.
BtTLP catalyzes N+1 nucleotide addition to 5′-truncated tRNAHis with enhanced catalytic efficiency over N-1 addition reactions. kcat/KM values are shown for BtTLP-catalyzed addition of each of the four possible Watson–Crick templated N−1 (solid bars) or N+1 (hatched bars) nucleotides to full-length tRNAHis or 5′-truncated tRNAHisΔN+1 substrates, respectively. For each of the four nucleotides (G, C, A or U, as indicated below the figure), kinetic parameters were measured using a tRNA substrate with the appropriate N73 or N72 residue to allow Watson–Crick base paired 3′–5′ addition, as in Table 1 and Supplementary Table S1. For comparison, the kcat/KM value measured previously for yeast Thg1-catalyzed G−1 addition to C73-tRNAHis is also shown (9).