2). impact of current antiviral therapies on viral persistence, particularly on cccDNA. The animal hepadnaviruses, together with the mammalian foamy viruses and plant caulimoviruses, take a special place within the group of viruses that replicate their genomes with the help of a reverse transcriptase (RT): their genomes are DNA, not RNA. These viruses synthesize DNA in the infected cell before the release of infectious, DNA-containing particles, in contrast to conventional retroviruses that perform DNA synthesis immediately following infection (Fig. 1). Another feature that sets the hepadnaviruses apart from even its closest relatives lies in the mechanism of RNA packaging and initiation of DNA synthesis, closely linked events that result in a covalent linkage between the first (minus) DNA strand and the RT. The DNA polymerase and RNase H activities encoded in the RT gene are the only known enzymatic functions specified by hepadnavirus genomes and are major targets for antiviral therapies relying on nucleoside analogs that can suppress, but not cure viral infections. The failure to cure hepatitis B virus (HBV) infections is a consequence of yet another unique property of the hepadnavirus life cycle: the intracellular amplification of the viral covalently closed circular DNA (cccDNA) and its apparent stability in nuclei of infected hepatocytes. Open in a separate window Figure 1. Hepadnavirus life cycle. The figure shows a model for the life cycle of hepadnaviruses RNA- and DNA-containing capsids are shown in red and blue, respectively. For simplicity, only pregenomic RNA (pgRNA) is shown. ccc, covalently closed circular; dsl, double-stranded linear; env, envelope; rc, relaxed circular. (From Seeger et al. 2013; adapted, with permission, from the authors.) FORMATION OF cccDNA The DNA genome of hepadnaviruses, a relaxed circular DNA (rcDNA), is held together by complementary overlaps that span the region between the 5 ends of the two DNA strands (Fig. 2). In mammalian hepadnaviruses, plus strands are shorter than minus strands and have heterogeneous 3 ends. In avian hepadnaviruses, plus-strand DNA synthesis is almost complete. Following infection of a hepatocyte, the first step in the viral DNA replication cycle is the conversion of rcDNA into cccDNA (Fig. 1). In addition to the extension of plus strands, this process requires removal of the RT from the 5 end Edrophonium chloride of minus-strand DNA and a primer comprising a capped, 18-nucleotide-long RNA derived from the 5 end of pgRNA (Figs. 2 and ?and3).3). In addition, one of the two 9-nucleotide-long terminally redundant segments on minus-strand DNA (r) (Fig. 3D) is removed and the two DNA strands are ligated resulting in the formation of cccDNA. Open in a separate window Figure 2. Hepatitis B virus (HBV) genome structure. The relaxed circular DNA Edrophonium chloride (rcDNA) genome of HBV with a complete minus strand (black) and incomplete plus strand (cyan) is shown, together with pregenomic RNA (pgRNA) (red) and the core and pol genes required for DNA replication. Reverse transcriptase (RT) and a capped RNA oligomer at the 5 Rabbit polyclonal to AKR7A2 ends of minus- and plus-strand DNA, respectively, are indicated. The positions of the start sites for the translation of the precore, presurface 1, -2, surface, and X proteins are marked by arrows (PC, PS1, PS2, S, and X, respectively). R, large terminal redundancy on pgRNA; DR, direct repeat; TP, terminal protein of RT. Open in a separate window Figure 3. DNA replication cycle. The figure shows a model for the formation of relaxed circular DNA (rcDNA) and double-stranded linear (dsl) from pregenomic RNA (pgRNA). (for encapsidation. J Virol 68: 2084C2090. [PMC free article] [PubMed] [Google Scholar]Cao F, Badtke MP, Metzger LM, Yao E, Adeyemo B, Gong Y, Tavis JE. 2005. Identification of an essential molecular contact point on the duck hepatitis B virus reverse transcriptase. J Virol 79: 10164C10170. [PMC free article] [PubMed] [Google Scholar]Cao F, Jones SA, Li W, Cheng X, Hu Y, Hu J, Tavis JE. 2013. Sequences in the terminal protein and reverse transcriptase domains of the hepatitis B virus polymerase contribute to RNA binding and encapsidation. J Viral Hepat Edrophonium chloride 21: 882C893. [PMC free article] [PubMed] [Google Scholar]Chang LJ, Hirsch RC, Ganem D, Edrophonium chloride Varmus HE. 1990. Effects of insertional and point mutations on the functions of the duck hepatitis B virus polymerase. J Virol 64: 5553C5558. [PMC free article] [PubMed].