Proofreading Function of Deoxyribonucleic Acid Polymerase I from Escherichia coli: NATURE OF EXCISION OF RIBONUCLEOTIDES FROM THE 3′ TERMINI OF OLIGODEOXYNUCLEOTIDE PRIMERS

Abstract

Abstract Oligothymidylic acid containing ribouridylic or riboadenylic acid residues at the 3' terminus was tested as primer for Escherichia coli DNA polymerase I and its large fragment in the presence of polydeoxyadenylic acid as template. Under conditions of synthesis, i.e. in the presence of deoxythymidinetriphosphate and Mg2+, ribouridylic acid residues are preserved during the extension of the primer in contrast to riboadenylic acid residues, which are excised by the 3' → 5' exonuclease activity. Thus, the error corrective function of DNA polymerase only discriminates mismatched from matched bases at the 3' terminus of the primer. Ribonucleotide residues are accepted as long as they allow classical base pairing with the template. In the absence of deoxythymidinetriphosphate, the 3' → 5' exonuclease activity is capable of cleaving both riboadenylic and ribouridylic acid residues from the 3' end of the primer.