Conservation of the HBV RNA element epsilon in nackednaviruses reveals ancient origin of protein-primed reverse transcription
View ORCID ProfileJürgen Beck, View ORCID ProfileStefan Seitz, View ORCID ProfileChris Lauber, and View ORCID ProfileMichael Nassal
Edited by Stephen P. Goff, Columbia University Medical Center, New York, NY, and approved February 7, 2021 (received for review October 26, 2020)
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Significance
Hepadnaviruses, including hepatitis B virus (HBV) as a major human pathogen, are small, enveloped DNA viruses, which replicate by reverse transcription of an RNA intermediate. Unlike retroviruses, which use tRNA as primer for reverse transcription, hepadnaviruses employ a unique protein-priming mechanism involving de novo synthesis of a DNA primer covalently attached to the viral polymerase. Here, we show that this mechanism is highly conserved on the molecular level in distantly related non-enveloped fish viruses which diverged from ancestral hepadnaviruses more than 400 Mya. The exceptional level of conservation and the absence of known homologous cellular mechanisms renders HBV protein priming a promising, yet unexplored, antiviral target for the development of novel therapeutics against this highly relevant pathogen.
Abstract
Hepadnaviruses, with the human hepatitis B virus as prototype, are small, enveloped hepatotropic DNA viruses which replicate by reverse transcription of an RNA intermediate. Replication is initiated by a unique protein-priming mechanism whereby a hydroxy amino acid side chain of the terminal protein (TP) domain of the viral polymerase (P) is extended into a short DNA oligonucleotide, which subsequently serves as primer for first-strand synthesis. A key component in the priming of reverse transcription is the viral RNA element epsilon, which contains the replication origin and serves as a template for DNA primer synthesis. Here, we show that recently discovered non-enveloped fish viruses, termed nackednaviruses [C. Lauber et al., Cell Host Microbe 22, 387–399 (2017)], employ a fundamentally similar replication mechanism despite their huge phylogenetic distance and major differences in genome organization and viral lifestyle. In vitro cross-priming studies revealed that few strategic nucleotide substitutions in epsilon enable site-specific protein priming by heterologous P proteins, demonstrating that epsilon is functionally conserved since the two virus families diverged more than 400 Mya. In addition, other cis elements crucial for the hepadnavirus-typical replication of pregenomic RNA into relaxed circular double-stranded DNA were identified at conserved positions in the nackednavirus genomes. Hence, the replication mode of both hepadnaviruses and nackednaviruses was already established in their Paleozoic common ancestor, making it a truly ancient and evolutionary robust principle of genome replication that is more widespread than previously thought.
protein priming initiation of reverse transcription HBV replication mechanism HBV long-term evolution-paleo virology作者: StephenW 时间: 2021-3-24 11:24