316회 Mechanism of Hepatitis B Virus Genome Replication

316회

연사 : 유 왕 식 , 연세대학교 생화학과

제목:  Mechanism of Hepatitis B Virus Genome Replication

Abstract

Hepatitis B viruses (HBV) are a family of viruses, also known as hepadnaviruses, that can cause disease of the liver, including cancer.  These enveloped viruses contain a 3.2-kbp circular DNA genome.  Although they have a DNA genome, hepadnaviruses replicate through reverse transcription of an RNA intermediate, (RNA pregenome) resulting in a relaxed circular DNA genome.  Synthesis of the relaxed-circular DNA genome by reverse transcription involves three template switches.  It is reported that direct repeat elements (i.e., DR1, DR2) are involved in these template switches during viral DNA genome synthesis.  These elements are known to act as either donor or acceptor during template switches.  On the other hand, it is not known whether sequences other than the previous known the donor and acceptor sites are involved in viral genome synthesis.  To map cis-elements, a series of mutants which contained a small deletion were made and analyzed for their requirement for viral genome synthesis.  We found that four regions of HBV genome are essential for the synthesis of the relaxed-circular DNA: named α, β, γ, and δ.   Apparently, these elements are involved in template switches during the plus-strand DNA synthesis.  

Hepatitis B virus (HBV) replicates by reverse transcription of an RNA intermediate, the pregenomic RNA.  The first step of HBV genome replication is the encapsidation of the pregenomic RNA encoding the encapsidation signal, termed, into the core particles.  The pregenomic RNA contains two identical  elements due to its terminal redundancy: one near the 5´ end and another near the 3´ end.  Despite that both elements form an indistinguishable secondary structure, only the 5´, but not the 3´, is functional for encapsidation.  We found that the cap-free lacZ RNA transcribed by T7 RNA polymerase was not encapsidated, implicating that the 5´ cap structure is required for encapsidation of the pregenomic RNA.  We hypothesized that HBV DNA polymerase must recognize the cap structure or its associated factors as well as the 5´  simultaneously for encapsidation to occur.