Human DNA polymerase δ (pol δ) is required for the synthesis of leading strand of simian virus 40 (SV40) DNA replication in vitro. Pol δ requires the accessory factors, proliferating cell nuclear antigen (PCNA), activator 1 (A1; also known as replication factor C [RF-C]), human single-stranded DNA binding protein (HSSB; also known as replication protein A [RP-A]) for the elongation of primed template DNA. Since pol δ has an associated 3′-5′ exonuclease activity, the effect of pol δ accessory factors on the exonuclease activity was examined. The 3′-5′ exonuclease activity was stimulated 8-10 fold by the addition of HSSB, and this stimulatory effect was preferential to HSSB since other SSBs from E.coil, T4 or adenovirus, had a little or no effect. The stimulatory effect of HSSB was markedly inhibited by the combined action of A1 and PCNA. Furthermore, the addition of deoxyribonucleoside trlphosphates (dNTPs) completely abolished the effect of HSSB on the 3′-5′ exonuclease activity even In the absence of pol δ accessory factors. These results suggest that accessory factors and dNTPs regulate both the polymerase and the 3′-5′ exonuclease activities.
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