Covalently closed circular (CCC) double-stranded DNA believed to be the transcriptional template for uck hepatitis B virus (DHBV) is amplified in aging primary cultures of hepatocytes from congenitally infected ducklings. Analysis of L-bromodeoxyuridinelabeled heavy/light CCC DNA shows that the relaxed circular DNA synthesized in the cytoplasm by reverse transcription is the predominant precursor to the amplified pool of nuclear viral CCC DNA, In vitro infection of uninfected hepatocyte cultures with DHBV demonstrates that a similar 50-fold amplification of CCC DNA occurs during an early stage in the infection before virus production. This amplification allows the establishment of a pool of transcriptional templates in the cell without the need for semiconservative replication or multiple rounds of infection. This process may acaunt for the ability of hepadnavirus-infected cells ersistently to produce virus particles in the absence of stable integration of viral DNA. adnaviruses are a family of DNA-containing viruses include human hepatitis B virus, woodchuck hepatitis virus (Summers et al., 1978), ground squirrel hepatitis virus (Marion et al., 1980) and duck hepatitis B virus (QHBV)(Mason et al., 1980). These viruses replicate primarily in hepatocytes in vivo (Summers, 1981) and they establish chronic infections in the cells of their respective hosts. The virion DNA of hepadnaviruses is a small double-stranded relaxed circular (FE) molecule approximately 3 kb in length that is held in a circular conformation by a short cohesive overlap between the Yends of the two DNA strands.

NA synthesis has been shown to occur in the cytoplasm of persistently infected cells. This DNA synthesis is carried out in viral nucleocapsid particles that are precursors of the virion core, and utilizes an RNA genome (“pregenome”) as a template (Summers and Mason, 1982). Pregenomes, transcribed in the nucleus, are assembled into replicative complexes that consist of the viral core~ olypeptide, a reverse transcriptase activity, and a protein that serves as the primer for minus-strand DNA synthesis. DNA minus strands are synthesized by reverse transcription of the pregenome, and the pregenome is degraded, leaving the minus-strand DNA free to serve as a template for plus-strand DNA synthesis. Completion of the plus strand results in the formation of the RC virion DNA, with