Influence of the small leader exons 2 and 3 on human immunodeficiency virus type 1 gene expression

The human immunodeficiency virus type 1 (HIV-1) uses an elaborate alternati ve splicing pattern for the generation of both the 1.8-kb as well as the 4- kb classes of mRNA, An additional diversity of transcripts in both classes is created by the optional inclusion of the small exons 2 and 3 in the lead er sequence. To analyze a possible influence of these leader exons on HIV-1 gene expression, several series of expression vectors with different leade rs were constructed, expressing either Rev and Env or a heterologous coding sequence, i.e., the chloramphenicol acetyl transferaser (CAT) ORF Transfec tion experiments of HeLa-T4(+) cells revealed for all series of constructs that mRNA as well as protein expression was stimulated by the presence of e xon 2 and reduced by axon 3. The function of the leader exons 2 and 3 is ne ither dependent on the regulatory proteins Tat or Rev nor on viral coding s equences. Neither transcription rates nor stability of polyadenylated RNAs were found to be responsible for the different levels of steady-state mRNA. When either axon 2 or 3 was inserted into a heterologous intron, processin g of the primary transcripts generated identical mRNA species while maintai ning the differences in axon 2/3-dependent mRNA steady-state levels. These results may be explained by exon-specific nuclear RNA degradation rates, as also indicated by results from an in vitro degradation assay using a HeLa nuclear extract. (C) 2001 Academic Press.