DIFFERENTIAL REGULATION OF MAJOR SURFACE PROMOTER IN HEPATITIS-B VIRUS

The major surface promoter of human hepatitis B virus can produce thre e distinct groups of S transcripts, The initiation sites of these tran scripts are in close proximity, Encompassing the ATG for the middle su rface protein, the largest S transcript (+1) encodes the middle surfac e protein whereas the other two (+20 and +31) can only code for small surface protein, Sequence analysis does not reveal any TATA element, I n this study, we employ deletion, linker scanning, and linker insertio n analyses to study systematically the sequence requirements for the i nitiations of all three transcripts and their upstream regulatory sequ ences, Our study reveals that the sequence downstream of -16 is suffic ient for precise initiation of all three groups of S transcripts, The 3' boundary of minimal promoter element is +15 for the +1 transcript, whereas it is +39 for both +20 and +31 transcripts, Furthermore, there are distinct sequence requirements for the initiations of three group s of S transcripts, The sequences from -17 to -10 and from -1 to +7 ar e required for the initiation of +1 transcript, the sequence from +16 to +39 is essential for the +20 transcript, and the sequences from -17 to -10 and from +24 to +39 are required for the +31 transcript, Our r esults also suggest that the transcription initiations of major surfac e promoter may be mediated in part by initiators, The initiations of t hese three groups of S transcripts are under differential regulation, The region from -39 to -16 containing both negative and positive regul atory elements selectively regulates the transcription levels of the t wo major S transcripts, Most notably, mutation of the sequence from -1 7 to -10, which contains a Sp1 site, leads to an increase in the impre cise initiation at +1 site and depresses the initiation of +20 and, to a greater extent, +31 transcript, The relevance of differential regul ation of major surface promoter to the varied production of different surface protein isoforms in viral life cycle is discussed.