An RNA stem-loop structure directs hepatitis B virus genomic RNA encapsidation

Area of Science:

• Virology
• Molecular Biology
• RNA Biology

Background:

• Selective encapsidation of hepatitis B virus (HBV) genomic RNA into core particles is essential for viral replication.
• This process is mediated by a cis-acting element known as epsilon (ε), located at the 5' end of the viral RNA.

Purpose of the Study:

• To map the minimal functional region of the HBV epsilon signal.
• To elucidate the secondary structure of the epsilon element.
• To investigate the role of specific structural features and sequences within epsilon in RNA encapsidation.

Main Methods:

• Transfection of plasmids expressing chimeric RNAs (HBV-lacZ fusions).
• Mapping of the minimal epsilon region using functional assays.
• Enzymatic probing to determine RNA secondary structure.
• Site-directed mutagenesis to assess the impact of mutations on encapsidation in vivo.

Main Results:

• The minimal epsilon region was mapped to the 5' 94 nucleotides of the HBV genomic RNA.
• Enzymatic probing revealed a stem-loop structure within epsilon, featuring a lower stem, a 6-nt bulge, an upper stem with a single unpaired U, and a 6-nt loop.
• Mutational analysis demonstrated that base pairing in the lower stem is critical, while specific sequences in the loop and upper stem are essential for efficient RNA encapsidation.

Conclusions:

• The secondary structure of the HBV epsilon signal is critical for selective RNA encapsidation.
• Specific sequence elements within the epsilon loop and upper stem are vital for function.
• Understanding these structural requirements can inform strategies targeting HBV replication.