IDENTIFICATION OF POSITIVE AND NEGATIVE SPLICING REGULATORY ELEMENTS WITHIN THE TERMINAL TAT-REV EXON OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1

The requirement of human immunodeficiency virus type 1 to generate num erous proteins from a single primary transcript is met largely by the use of suboptimal splicing to generate over 30 mRNAs, To ensure that a ppropriate quantities of each protein are produced, there must be a si gnal(s) that controls the efficiency with which any particular splice site in the RNA is used. To identify this control element(s) and to un derstand how it operates to generate the splicing pattern observed, we have initially focused on the control of splicing of the tat-rev intr on, which spans the majority of the env open reading frame. Previous a nalysis indicated that a suboptimal branchpoint and polypyrimidine tra ct in this intron contribute to its suboptimal splicing (A. Staffa and A. Cochrane, J. Virol. 68:3071-3079, 1994). In this report, we identi fy two additional elements within the 3'-terminal exon, an exon-splici ng enhancer (ESE) and an exon splicing silencer (ESS), that modulate t he overall efficiency with which the 3' tat-rev splice site is utilize d. Both elements are capable of functioning independently of one anoth er, Furthermore, while both the ESE and ESS can function in a heterolo gous context, the function of the ESS is extremely sensitive to the se quence context into which it is placed. In conclusion, it would appear that the presence of a suboptimal branchpoint and a polypyrimidine tr act as well as the ESE and ESS operate together to yield the balanced splicing of the tat-rev intron observed in vivo.