The amino-terminal domain of the hepadnaviral P-gene encodes the terminal protein (genome-linked protein) believed to prime reverse transcription.

A series of antisera directed against amino acid sequences from different segments of the duck hepatitis B virus (DHBV) P-gene were shown to immunoprecipitate DHBV DNA molecules that were covalently linked to the DHBV DNA terminal protein. Restriction analysis and sizing after protease treatment demonstrated that the P-gene proteins were bound to the 5'-end of the DHBV DNA minus-strand which was mapped to a G-residue in the centre of the repeat sequence DR1. Resistance to alkali treatment indicated a phosphodiester linkage to tyrosine between protein and DNA. Limited protease treatment prior to immunoprecipitation cleaved C-terminal P-proteins from the viral DNA, indicating that the terminal protein forms a separate domain encoded in the N-terminal part of the P-gene. Functional analysis of a deletion mutant confirmed the notion that a non-essential spacer separates the terminal protein from the polymerase domain residing in the C-terminal half of the P-gene. Thus, the major proteins required for hepadnaviral reverse transcription, namely the primer, DNA polymerase, and possibly also RNase H, appear to be synthesized as a polyprotein precursor which is at least initially linked as such to its first DNA product.