The gene S promoter of hepatitis B virus confers constitutive gene expression

The properties of the promoter of the hepatitis B surface antigen (HBsAg) were studied using recombinants containing either this promoter or the SV40 early promoter. Mouse L cells were transfected with these recombinants and the levels of gene expression obtained with the two promoters were compared. The level of expression of a cellular gene, the human fibroblast interferon gene, obtained with the HBsAg promoter was comparable to that obtained with the SV40 early promoter. Similarly when the HBsAg gene was controlled by the SV40 early promoter the level of HBsAg synthesis is in the same range as that observed with its own promoter. Together these results suggest that although the HBsAg gene codes for a structural viral protein, its expression is constitutive as for an early gene. The implications of these observations on the synthesis of HBV particles in vivo are discussed.