Although efficient vaccines are available, chronic hepatitis B (HBV) infection poses a major health problem worldwide, and prolonged treatment of chronically infected HBV patients with nucleoside analogs often results in drug-resistant HBV variants. Therefore, it is critical to evaluate the contribution of the HBV polymerase to mutations. FLAG-tagged wild-type (FPolE) and mutant (FPolE/D551A) HBV polymerases have been expressed in insect cells and purified. The purified FPolE showed DNA polymerase activity, but FPolE/D551A did not, implying that the activity was derived from FPolE. No 3'-->5'exonuclease activity was detected in FPolE. The fidelity of FPolE was investigated and compared with that of HIV-1 RT, which is highly error-prone. The fidelity of HBV polymerase seems to be achieved by increasing the Km for the dNTP being misinserted. The nucleotide misinsertion efficiency of FPolE and HIV-1 RT ranged from 3.59 x 10-4 (C : T) to 1.51 x 10-3 (G : T) and from 1.75 x 10-4 (C : T) to 1.62 x 10-3 (G : T), respectively, and the overall misinsertion efficiency of HIV-1 RT was just 1.04-fold higher than that of FPolE, implying that HBV polymerase is fairly error-prone. Though HBV genetic mutation rate in replication is thought to be between those in RNA and DNA viruses, our data shows that the rate of mutation by HBV polymerase is higher than the rate of genetic mutation in vivo. This may be a result from more overlapping HBV genes in the HBV genome than that of other retroviruses.
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