Characterization of hepatitis B virus X gene: in vitro translation of mRNA from COS-1 cells transfected with the X gene.

To elucidate the biological role of the X region of HBV DNA, we constructed an expression plasmid pSV2HBx by inserting the StuI/BglII fragment of HBV DNA into the pSV2 vector. Although X region products were not identified in vivo in transfected COS-1 cells, transcripts were translated in vitro and a 17-k Da protein was immunoprecipitated with an antibody raised against a synthetic peptide corresponding to the carboxy terminus of the X region open reading frame. Among 68 HBV carrier sera tested, two were found to contain antibodies reactive to the in vitro translation product. These results suggest that the X region is a gene which can be actually expressed at some stage of HBV infection.