Objective: To construct a co-expression vector which expresses both HBsAg and Tet Repressor protein and in which the expression of HBsAg can be regulated by tetracycline

and to study the regulating role of tetracycline in the HBV-S expression in HepG 2 cell.Methods:Tetracycline repressor(TetR)gene was insert into the first multiple cloning site

which was just after the first promotor in pVITRO3

a co-expression vector

then the second promotor was partly replaced by another one which contained tetracycline operator.Then

HBs gene was inserted into the second multiple cloning site.The constructed vector was transfected into HepG2 by means of lipofectamine.And 400 μg/ml Hygromycin B was used to kill the untransfected host cell line.After 20 days selection

the cell line which expressed HBsAg stably after being induced by tetracycline was generated.Then the HBsAg expression was induced by various concentrations of tetracycline(0.5

1.0

2.0

3.0 μg/ml).And HBsAg was detected by means of RT-PCR and MEIA(Microparticle Enzyme Immunoassay)to determine the appropriate concentration of tetracycline.Results:HBsAg was expressed little without tetracycline in transfected HepG2.The average S/N in 72 hour was 1.57.And the S/N of HBsAg was increased remarkably but had no difference after being induced by various concentrations of tetracycline.The average S/N in 72 hour was 4.24

4.23

4.18 and 4.20 respectively.Conclusion:The basal expression was very low in the tetracycline regulated expression vector of HBV-S gene without inducer.After being induced by various concentrations of tetracycline

HBsAg expression was sharply increased with the incubation time prolongation

but the expression quantity showed no difference at the same time point.