The function of 3' noncoding sequences of hepatitis B surface antigen gene in a non-hepatic cell expression system

A series of expression vectors containing the hepatitis B surface antigen (HBsAg) gene were constructed in which the promoter region of the HBsAg gene was replaced by the mouse metallothionein promoter. The HBsAg 3' noncoding sequences were modified by various deletions and/or additions of the heterogeneous genetic elements. The expression levels of the derivates were evaluated in a transient expression system using HeLa cells. We found that the enhancer I (EN-I) of hepatitis B virus (HBV) and the polyadenylation (poly A) signal are the most important elements for the expression of HBsAg gene in non-hepatic cells, in addition to the necessity to replace the original promoter of HBsAg gene. The heterogeneous poly A signal in combination with the simian virus (SV40) splicing signal could substitute for the poly A signal of the HBsAg gene, and, moreover, could increase the expression level of the HBsAg gene in HeLa cells. On the contrary, the enhancer II (EN-II) of HBV has no effect on the expression of the HBsAg gene in HeLa cells. These results imply another role of HBV EN-II on the stability of HBsAg transcripts in addition to the demonstrated function of activation the promoter.