ACCURACY OF DEOXYNUCLEOTIDE INCORPORATION BY SOYBEAN CHLOROPLAST DNA-POLYMERASES IS INDEPENDENT OF THE PRESENCE OF A 3' TO 5' EXONUCLEASE

DNA polymerase was purified from soybean (Glycine max) chloroplasts th at were actively replicating DNA. The main form (form I) of the enzyme was associated with a low level of 3' to 5' exonuclease activity thro ughout purification, although the ratio of exonuclease to polymerase a ctivity decreased with each successive purification step. A second for m (form II) of DNA polymerase, which elutes from DEAE-cellulose at a h igher salt concentration than form I, was devoid of any exonuclease ac tivity. To assess the potential function of the 3' to 5' exonuclease i n proofreading, the fidelity of deoxynucleotide incorporation was meas ured for form I DNA polymerase throughout purification. Despite the st eadily decreasing ratio of 3' to 5' exonuclease to polymerase activity , the extent of misincorporation by form I enzyme remained unchanged d uring the final purification steps, suggesting that the exonuclease di d not contribute to the accuracy of DNA synthesis by this polymerase. Fidelity of form I DNA polymerase, when compared with that of form II, revealed a higher level of misincorporation for form I enzyme, a find ing that is consistent with the exonuclease playing little or no role in exonucleolytic proofreading.