Identification of a doubly spliced viral transcript joining the separated domains for putative protease and reverse transcriptase of hepatitis B virus.

Hepatitis B virus (HBV), like retroviruses, replicates through reverse transcription. However, the identity and mechanism for the synthesis of HBV reverse transcriptase remain unknown. The open reading frame (ORF) for HBV putative reverse transcriptase (pol), as a consequence of overlapping with the whole ORF of envelope proteins (hepatitis B surface antigens), includes a hypervariable region at the N terminus. Thus, compared with retroviruses, it is unlikely that HBV reverse transcriptase is translated from complete pol ORF in the full-length pregenomic RNA. We have now detected in infected human livers a novel doubly spliced RNA in which one splicing event removed the hypervariable region of the pol gene but retained the conserved region homologous to retroviral reverse transcriptase. The other splicing event deleted the central region of hepatitis B core antigen and thus brought the protease domain which is important for maturation of reverse transcriptase close to that of pol. For this sequence organization, the spliced RNA as the possible template for the synthesis of HBV reverse transcriptase is discussed.