Hepatitis B viruses: reverse transcription a different way.

Hepatitis B virus (HBV), the causative agent of B-type hepatitis in humans, is the type member of the Hepadnaviridae, hepatotropic DNA viruses that replicate via reverse transcription. Beyond long-established differences to retroviruses in gene expression and overall replication strategy newer work has uncovered additional distinctions in the mechanism of reverse transcription per se. These include protein-priming by the unique extra terminal protein domain of the reverse transcriptase (RT) utilizing an RNA hairpin for de novo initiation of first strand DNA synthesis, and the strict dependence of this process on cellular chaperones. Recent in vitro reconstitution systems enabled first biochemical insights into this multifactorial reaction, complemented by high resolution structural information on the RNA, though not yet the protein, level. Genetic approaches have revealed long-distance interactions in the nucleic acid templates as an important factor enabling the puzzling template switches required to produce the relaxed circular (RC) DNA found in infectious virions. Finally, the failure of even potent HBV RT inhibitors to eliminate nuclear covalently closed circular (ccc) DNA, the functional equivalent of integrated proviral DNA, has spurred a renewed interest in the mechanism of cccDNA generation. These new developments are in the focus of this review.