A novel cis-acting element facilitates minus-strand DNA synthesis during reverse transcription of the hepatitis B virus genome

Hepadnaviruses replicate through reverse transcription of an RNA pregenome, resulting in a relaxed circular DNA genome. The first 3 or 4 nucleotides (nt) of minus-strand DNA are synthesized by the use of a bulge in a stem-loop structure near the 5′ end of the pregenome as a template. This primer is then transferred to a complementary UUCA motif, termed an acceptor, within DR1* near the 3′ end of the viral pregenome via 4-nt homology, and it resumes minus-strand DNA synthesis: this process is termed minus-strand transfer or primer translocation. Aside from the sequence identity of the donor and acceptor, little is known about the sequence elements contributing to minus-strand transfer. Here we report a novel cis-acting element, termed the β5 region (28 nt in length), located 20 nt upstream of DR1*, that facilitates minus-strand DNA synthesis. The deletion or inversion of the sequence including the β5 region diminished minus-strand DNA synthesis initiated at DR1*. Furthermore, the insertion of the β5 region into its own position in a mutant in which the sequences including the β5 region were replaced restored minus-strand DNA synthesis at DR1*. We speculate that the β5 region facilitates minus-strand transfer, possibly by bringing the acceptor site in proximity to the donor site via base pairing or by interacting with protein factors involved in this process.