proofreading activity conferred by the exonuclease domain was demonstrated by showing the preferential excision of a mismatched nucleotide from the 3' terminus of the newly synthesised strand. the exonuclease activity of the enzyme is, like its polymerisation activity, highly processive and can degrade single stranded oligonucleotides without dissociation. co-operation or a 'delicate competition' between these two functional domains is essential, so as to ensure accurate elongation at an optimal rate. the exonuclease activity of the enzyme does impede its polymerisation capacity; inactivation of the exonuclease activity by site-directed mutagenesis meant that a 350 fold lower dntp concentration was required to achieve the same rates of primer elongation seen in the wild type enzyme.