THE HEPATITIS-B VIRUS POSTTRANSCRIPTIONAL REGULATORY ELEMENT CONTAINS2 CONSERVED RNA STEM-LOOPS WHICH ARE REQUIRED FOR FUNCTION

Human Hepatitis B Virus (HBV) RNAs contain a cis-acting sequence, the post-transcriptional regulatory element (HPRE), which facilitates the cytoplasmic localization of intronless transcripts. Our previous studi es have shown that the HPRE is composed of at least two independent su b-elements, HPRE alpha and HPRE beta, which co-activate a reporter for RNA export in a greater than additive manner. Utilizing deletion, mut ation and co-variational analyses, we have identified th ree reg ions important for full HPRE activity. The th ree separate regions of the H PRE function can function independently in a dose-dependent manner whe n multimerized. Two of these regions contain stem loops, HSL alpha and HSL beta 1, which are necessary for full HPRE function, These structu res are conserved throughout the mammalian Hepadnaviruses, Disruption of either stem-loop structure by mutagenesis decreases HPRE function w hile compensatory mutations restore activity. The location of the stem -loops in the genome reveal that they are present in all of the HBV tr anscripts, HSL alpha and HSL beta 1 are likely to contain the binding sites for the cellular factor(s) which mediates HPRE function.