Elimination of template-independent base addition by mutant Tne DNA polymerases. (A) The substrate with five randomized nucleotides at the 5′-end of a 48 nt template used in the assay. (B) An autoradiograph of the reaction products. The assay was carried out as described in Materials and Methods with 1 U of Tne DNA polymerase and 100 µM dNTPs. All DNA polymerases used in this experiment are deficient in both 5′→3′ exonuclease (D137A) and 3′→5′ exonuclease (D323A) activities. The results with D137A/D323A, R722K, R722Y, R722L, R722H, R722Q and F730Y are shown in lanes 1–7, respectively. The unextended primer in the reaction mixture without DNA polymerase is shown in lane 0. The arrow indicates extended 48 nt product without template-independent base addition to the 3′-end. The band above the arrow represents the extra nucleotide addition (n + 1) product.