sequencing dna using the dna polymerase as well as a kit for sequencing dna is also disclosed application_number US 33481899 A description a series of polymerase mutants were constructed with the aim of obtaining an improved polymerase for dna sequencing by reducing the exonuclease activity found in full length thermus thermophilus and thermus aquaticus dna polymerase i enzymes six conserved motifs gutman and minton 1993 nucleic acids research 21 4406 4407 can be identified in the amino terminal domain of pol i type polymerases in which the 5 u2032 to 3 u2032 exonuclease activity has been shown to reside further six carboxylate residues in these conserved regions have been shown in a crystal structure to be located at the active site of the exonuclease domain of thermus aquaticus dna pol i kim et al 1995 nature 376 612 616 point mutations were made by site directed mutagenesis to carboxylates and other residues in three of six conserved motifs in tth and taq polymerases as follows taq d18a taq t140v taq d142n d144n all of these have the mutation f667y outside of the exonuclease domain tth d18a tth t141v tth d143n d145n all of these have the mutation f669y outside of the exonuclease domain all polymerases were evaluated for exonuclease activity processivity strand displacement salt tolerance thermostability and sequencing quality one fy7 polymerase tth d18a.