Biopolymers and cell. 1999; 15 (2): 109 - 121

The hepatitis B virus: general description, physical structure, genetic organization, gene transcripts arid genomic regulatory elements o ;

Farzam Adjamian

The HBV genome is a circular partially double-stranded DNA molecule. It contains four overlapping open reading frames (ORFs) genes. The S (preSJ, prcS2) region(s) encodes the major (small), middle and large proteins (HBsAg). The C and. pre-C regions encode HBeAg and HBeAg. The X region encodes a polypeptide expressed during HBV infection. The P region codes for a protein with several, functions in replication. Four classes of HBV mRNAs have been identified, in the HBV genome the pre-Sl promoter expresses the large protein. The pre-S2/S promoters produce both major and middle proteins. The X promoter produces 0.7 and 0.9 kb transcripts. The C and pre-C promoters produce the Core, pol(HBcAg), and the HBeAg proteins. Enhancer I and II are key regulatory elements in the transcriptional regulation of HBV. The activity of enhancer II is highly the liwr specific. Enhancer II activates the transcriptional activity of both the pre-Sl and preS2/S promoters. Two HBV enhancers strongly affect the activity of all three major HBV promoters. A box-a in the II-A and box-fi in JI-B elements of HBV genome, are necessary for the enhancer II function. Either box-a or box-ft can Kgulate the activity of the Core promoter, a, b, f proteins and c, d proteins bind to box-a and box-fS, respectively, and mediate the enhancer function.