Cryptic proofreading 3'. -->. 5' exonuclease is associated with the catalytic subunit of DNA polymerase-. cap alpha. from Drosophila melanogaster

Purified DNA polymerase-..cap alpha.., the major replicative polymerase in eucaryotic cells, lacks an associated 3' ..-->.. 5' exonuclease activity. In procaryotes, exonucleolytic proofreading enhances the accuracy of DNA synthesis by 30- to 100-fold. They confirmed that the DNA polymerase-..cap alpha.. from Drosophila lacks detectable exonuclease activity. However, a 3' ..-->.. 5' exonuclease can be detected after separating the 182,000 MW catalytic subunit from other subunits of the enzyme by glycerol gradient sedimentation in the presence of 50% ethylene glycol. The exonuclease activity co-sediments with the 182 kD polymerase subunit, suggesting that both activities reside in the same polypeptide. The 3' ..-->.. 5' exonuclease excises mismatched bases at the 3' termini of double-stranded DNA templates. Excision of a complementary nucleotide at the 3' terminus occurs at a 10-fold lower frequency. When assayed using the Phi X fidelity assay, the error rate of the isolated catalytic subunit is 10/sup -7/; 100 times more accurate than the complex prior to disassociating or a complex of the 182 kD + 73 kD subunits. This parallelism between activation of the 3' ..-->.. 5' exonuclease and increased fidelity suggests that this exonuclease increases accuracy as it does with procaryotic polymerases.