Proofreading by the epsilon subunit of Escherichia coli DNA polymerase III increases the fidelity of calf thymus DNA polymerase alpha.

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Abstract
Addition of the 3'' .fwdarw. 5'' proofreading exonuclease, .epsilon. subunit of Escherichia coli DNA polymerase III, to DNA polymerase .alpha. from calf thymus has been studied. Alone, calf thymus DNA polymerase .alpha. terminates in vitro DNA synthesis upon insertion of noncomplementary nucleotides. Upon addition of the .epsilon. subunit, DNA polymerase .alpha. elongates the newly synthesized DNA as a result of hydrolysis of the 3''-terminal mispair. The fidelity of DNA polymerase .alpha. in vitro is increased 7-fold by addition of the exonuclease. The functional interaction between DNA polymerase .alpha. and the .epsilon. subunit is independent of any detectable physical association. This suggests that a mechanism of proofreading could exist in mammalian cells involving sequential catalysis by DNA polymerase .alpha. excision of errors by a separate 3'' .fwdarw. 5'' exonuclease, and further elongation onto correctly base-paired 3'' termini by DNA polymerase .alpha.