Three novel cis-acting elements required for efficient plus-strand DNA synthesis of the hepatitis B virus genome

Synthesis of the relaxed-circular (RC) DNA genomes of hepadnaviruses by reverse transcriptase involves two template switches during plus-strand DNA synthesis. These template switches require repeat sequences (so-called donor and acceptor sites) between which a complementary strand of nucleic acid is transferred. To determine cis-acting elements apart from the donor and acceptor sites that are required for plus-strand RC DNA synthesis by hepatitis B virus (HBV), a series of mutants bearing a small deletion were made and analyzed for their impact on the viral genome synthesis. We found three novel cis-acting elements in the HBV genome: one element, located in the middle of the minus strand, is indispensable, whereas the other two elements, located near either end of the minus strand, contribute modestly to the plus-strand RC DNA synthesis. The data indicated that the first element facilitates plus-strand RNA primer translocation or subsequent elongation during plus-strand RC DNA synthesis, while the last two elements, although distantly located on the minus strand, act at multiple steps to promote plus-strand RC DNA synthesis. The necessity of multiple cis-acting elements on the minus-strand template reflects the complex nature of hepadnavirus reverse transcription.