Chimeras of Duck and Heron Hepatitis B Viruses Provide Evidence for Functional Interactions between Viral Components of Pregenomic RNA Encapsidation

Abstract

Packaging of hepadnavirus pregenomic RNA (pgRNA) into capsids, or encapsidation, requires several viral components. The viral polymerase (P) and the capsid subunit (C) are necessary for pgRNA encapsidation. Previous studies of duck hepatitis B virus (DHBV) indicated that two cis-acting sequences on pgRNA are required for encapsidation: , which is near the 5 end of pgRNA, and region II, located near the middle of pgRNA. Later studies suggested that the intervening sequence between these two elements may also make a contribution. It has been demonstrated for DHBV that interacts with P to facilitate encapsidation, but it is not known how other cis-acting sequences contribute to encapsidation. We analyzed chimeras of DHBV and a related virus, heron hepatitis B virus (HHBV), to gain insight into the interactions between the various viral components during pgRNA encapsidation. We learned that having and P derived from the same virus was not sufficient for high levels of encapsidation, implying that other viral interactions contribute to encapsidation. Chimeric analysis showed that a large sequence containing region II may interact with P and/or C for efficient encapsidation. Further analysis demonstrated that possibly an RNA-RNA interaction between the intervening sequence and region II facilitates pgRNA encapsidation. Together, these results identify functional interactions among various viral components that contribute to pgRNA encapsidation.Hepadnaviruses, also known as hepatitis B viruses, are DNA viruses that replicate via reverse transcription of an RNA pregenome (pgRNA) (9). The pgRNA is transcribed from the hepadnaviral plasmid present in the nucleus, called covalently closed circular DNA (5, 28). The pgRNA can be translated to produce the viral polymerase, P, and the capsid subunit, C (8, 26). Within the cytoplasm, the pgRNA and P are packaged into capsids in a process known as RNA encapsidation (1, 11). Capsids containing pgRNA and P are competent for reverse transcription of the pgRNA (27). Once DNA synthesis is complete, capsids can be secreted from the cell to produce virions.There are several viral components important for pgRNA encapsidation (Fig. 1). The C protein is the subunit of the capsid that contains pgRNA. C protein also might contribute to pgRNA encapsidation directly (21). The P protein is required for pgRNA encapsidation, but the reverse transcriptase or DNA-priming activity of P is not required (1,11,29,30). Also, the pgRNA contains cis-acting sequences that contribute to its encapsidation (6, 12, 16). One sequence, called ε, is located near the 5Ј end of the pgRNA. ε forms a phylogenetically conserved secondary structure that is important for its function (16,23,24). For the human hepatitis B virus (HBV), ε is the only identified sequence on the pgRNA required for encapsidation (16).For the avian hepadnaviruses, a second sequence located about 900 nucleotides (nt) downstream of ε, called region II, is required for encapsidation (6,12). Analysis of duck hepatitis B virus (DHBV) region II sho...

A High Level of Mutation Tolerance in the Multifunctional Sequence Encoding the RNA Encapsidation Signal of an Avian Hepatitis B Virus and Slow Evolution Rate Revealed by In Vivo Infection

“…Perhaps more importantly, in cell culture various mutations within cis elements had gradual rather than massive effects, uncovered only by thorough quantitative assessment (21,33,34,44). The drastic impact of the pet element on pgRNA accumulation was discovered using large deletions (26).…”

“…1A); in avian HBVs, this requires an additional, far-downstream cis element ("region II" (11,44)). The second crucial function mediated by the P-ε interaction is replication initiation itself (priming).…”

Investigating the hepatitis B virus life cycle using engineered reporter hepatitis B viruses

“…HBV does not require another sequence in addition to the sequence termed “epsilon” for the encapsidation of pgRNA . However, duck hepatitis B virus (DHBV) requires the epsilon sequence as well as its downstream sequence for encapsidation . This suggests that the mechanism of the encapsidation of pgRNA may be influenced by the downstream sequence of epsilon in the HBV genome and may affect the efficiency of encapsidation.…”

“…19 However, duck hepatitis B virus (DHBV) requires the epsilon sequence as well as its downstream sequence for encapsidation. 20 This suggests that the mechanism of the encapsidation of pgRNA may be influenced by the downstream sequence of epsilon in the HBV genome and may affect the efficiency of encapsidation. Our reporter HBV lacked some sequences between epsilon and the N-terminal region of the pol gene.…”

Few basepairing-independent motifs in the apical half of the avian HBV ε RNA stem-loop determine site-specific initiation of protein-priming

“…1C ) in protein and RNA 3 – 6 , and triggers co-packaging of pgRNA and P protein into newly forming nucleocapsids, i.e. encapsidation (in DHBV facilitated by the downstream “region II”) 7 , 8 , yet also the synthesis of a 3–4 nucleotide (nt) DNA oligo that is templated by the 3′ half of the ε bulge; its first nt is covalently linked to a Tyr-residue in P protein’s Terminal Protein (TP) domain (protein-priming). For extension into full-length minus-strand DNA the oligo is translocated to a complementary acceptor at the 3′ proximal direct repeat DR1* (Fig.…”