We have constructed two simian virus 40 early replacement recombinants that have the coding sequences for hepatitis B virus surface antigen (HBsAg). One construction, LSV-HBsAg, has the coding region for HBsAg but not the portion encoding the putative pre-surface antigen leader. Transformed monkey kidney cells (COS) infected with this recombinant express large quantities of the characteristic partially glycosylated HBsAg molecule, which are assembled into 22-nm particles that appear similar to those produced by human liver cells infected with hepatitis B virus. This result indicates that the pre-surface antigen sequences are not required for the synthesis of HBsAg or its assembly into particulate structures. The second recombinant, LSV-HBpresAg, has the entire surface antigen gene, including the putative promoter and pre-surface antigen region. COS cells infected with this recombinant plasmid produce 40- to 50-fold less HBsAg than those infected with the LSV-HBsAg recombinant plasmid. RNA mapping studies suggest that the transcription of the HBsAg gene is initiated at more than one site, or alternatively, that RNA splicing of transcripts occurs in the pre-surface antigen region.
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