2 STRONG 5'-SPLICE SITES AND COMPETING, SUBOPTIMAL 3'-SPLICE SITES INVOLVED IN ALTERNATIVE SPLICING OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 RNA

The human immunodeficiency virus type 1 (HIV-I) genome contains 20 exo ns that are alternatively spliced from 16 splice sites to generate mor e than 40 different mRNAs, including incompletely spliced and unsplice d mRNAs. In contrast to avian retroviral RNA, which has a cis-acting e lement in gag that negatively regulates splicing (NRS), HIV-I RNA did not have any NRS sequences in the gag or pol genes detectable by a spl icing inhibition assay. However, this assay demonstrated that the HIV- 1 first 5' splice site competed with a cellular 5' splice site, sugges ting that HIV-1 may have some strong splice sites. To extend this obse rvation, we used a splice site swapping strategy to determine the effi ciency of 14 HIV-I splice sites in human beta globin chimeras tested i n transient transfection experiments. While the Ist HIV-1 5' splice si te used in all spliced transcripts and the 4th 5' splice site used in most of the 2-kb transcripts were efficient, the other splice sites, i ncluding all the 3' splice sites; were less efficient, ranging in use from 25 to 60%. We propose that this range of splice site efficiencies contributes to the regulation of alternative splicing of HIV-1 mRNAs. (C) 1995 Academic Press, Inc.