X-region-specific transcript in mammalian hepatitis B virus-infected liver.

In vitro gene expression systems for hepatitis B virus have demonstrated that the virus genome is capable of producing an X-region-specific transcript of approximately 0.7 kilobases (kb). However, this transcript has not been detected in virus-infected cells. We now report the presence of a heterogeneous X-region-specific transcript of approximately 0.65 kb that is found primarily in the nucleus of liver cells infected with the woodchuck hepatitis virus. Interestingly, the majority of the transcripts are not polyadenylated. The transcript, which represents less than 1% of total virus-specific RNA, is found in animals with both acute and chronic woodchuck hepatitis virus infections. While it is probable that the 0.65-kb transcript is involved in the expression of the X gene protein, it may also direct the translation of a protein encoded by a newly identified open reading frame, ORF5, that is present in all hepadnavirus genomes analyzed.