The duck hepatitis B virus DNA polymerase is tightly associated with the viral core structure and unable to switch to an exogenous template.

The duck hepatitis B virus (DHBV) has a DNA polymerase associated with it which uses the incomplete viral genome as endogenous template. A prerequisite for studying this polymerase is the availability of conditions to open viral cores without destroying their enzymatic activity. In this study, this was achieved by a brief treatment with low pH. DHBV DNA in low-pH-treated cores was susceptible to digestion with deoxyribonuclease I and restriction enzymes, and large restriction fragments diffused out of the viral cores. However, the DHBV polymerase remained tightly associated with its DNA template in the viral core structure and could still incorporate nucleotides into those DNA fragments which carried the DNA-bound protein and remained in the core. The DHBV polymerase could not switch to any of several exogenously supplied templates although these were most likely accessible to it. The manner in which this tight association of the DHBV polymerase with the core may occur, and the possible implications of this interaction during the DHBV replication cycle, is discussed.