Transcription of the hepatitis B surface antigen gene in cultured murine cells initiates within the presurface region.

Cloned hepatitis B virus (HBV) DNA directs the synthesis of the viral surface antigen (HBsAg) when introduced into mouse L cells by DNA transformation. We have used recombinants between the Rous sarcoma virus long terminal repeat and subgenomic fragments of HBV DNA to localize regions of the HBV genome required for HBsAg expression. Examination of HBV-specific RNA from such transformants indicates that transcription initiates at three distinct sites (153, 163, and 183 nucleotides upstream from the translation initiation codon for mature HBsAg). Thus in these cells, a large segment of the presurface reading frame is not represented in HBsAg mRNA. The termination site of this RNA lies within the coding sequences for the viral core antigen, some 1,094 +/- 10 base pairs downstream from the TAA stop codon for HBsAg. Two additional open reading frames are present in the resultant unspliced HBsAg RNA.