Hepatitis B virus RNA element that facilitates accumulation of surface gene transcripts in the cytoplasm

Hepatitis B virus enhancers I and II are critical for high-level expression from the viral major surface gene promoter. These enhancers are in an unusual position, since both are entirely contained within the downstream transcribed region of the surface gene. In this report, we present data showing that a fragment of the viral genome encompassing enhancer II activates accumulation of surface gene transcripts at the posttranscriptional level. Specifically, the total steady-state amount of surface gene transcripts in the cell drops by more than fourfold when enhancer II is displaced to a position downstream of the transcription termination site. There is a similar decrease in the amount of cytoplasmic surface gene transcripts but not of nuclear transcripts. These changes in steady-state transcript levels do not result from a decrease in the rate of transcriptional initiation or from an increased rate of degradation in the cytoplasm. Reinsertion of enhancer II in the correct orientation into the surface gene transcribed region partially restores transcript levels. From these data, we conclude that a hepatitis B virus RNA element functions in cis to increase the steady-state levels of surface gene transcripts by facilitating cytoplasmic accumulation of these transcripts.