Deoxyribonucleic Acid Polymerase : Two Distinct Enzymes in One Polypeptide 11 . A PROTEOLYTIC FRAGMENT CONTAINING THE 5 ‘ 3 3 ’ EXONUCLEASE FUNCTION . RESTORA - TION OF INTACT ENZYME FUNCTIONS FROM THE TWO PROTEOLYTIC FRAGMENTS

The small fragment (mol wt 36,000) produced by the limited proteolytic cleavage of DNA polymerase (mol w t 109,000) retains only the 5’ + 3’ exonuclease activity. The small fragment resembles the 5’ -+ 3’ exonuclease of the intact enzyme in degrading DNA to monoand oligonucleotides and in its capacity to excise mismatched regions such as thymine dimers. It differs from the intact enzyme in that deoxyribonucleoside triphosphates, which support polymerization, fail to stimulate the exonuclease or to increase the proportion of oligonucleotides among the products. However with a mixture of small fragment, large fragment (mol wt 76,000; polymerase and 3‘ .-* 5’ exonuclease functions), nicked DNA, and suitable deoxyribonucleoside triphosphates, the same influences of polymerization on 5’ -+ 3’ exonucleases are seen as with the intact enzyme. Thus at the locus of a nick in DNA, the two fragments bind adjacent to one another to perform the coordinated polymerization-5’ -+ 3’ exonuclease functions that characterize the intact enzyme.