In vivo activity of the hepatitis B virus core promoter: tissue specificity and temporal regulation.

The contribution of the hepatitis B virus enhancers I and II in the regulation of the activity of the core and the X promoters was assessed in transgenic mice. Surprisingly, despite the presence of heterologous promoters linked 5' of the X gene, the transgene expression is mostly due to core promoter (Cp) activity present in the X coding sequence. Moreover, the restriction of Cp activity to hepatic tissue required the combined action of both enhancers I and II, whereas the proximity of these two enhancers was insufficient to confer tissue specificity on Xp activity. Furthermore, the liver-specific activity of the Cp was developmentally regulated in an enhancer I-independent manner.