ABSTRACT
The human immunodeficiency virus type 1 (HIV-1) uses an elaborate alternative 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 leader sequence. To analyze a possible influence of these leader exons on HIV-1 gene expression, several series of expression vectors with different leaders were constructed, expressing either Rev and Env or a heterologous coding sequence, i.e., the chloramphenicol acetyl transferase (CAT) ORF. Transfection experiments of HeLa-T4(+) cells revealed for all series of constructs that mRNA as well as protein expression was stimulated by the presence of exon 2 and reduced by exon 3. The function of the leader exons 2 and 3 is neither dependent on the regulatory proteins Tat or Rev nor on viral coding sequences. Neither transcription rates nor stability of polyadenylated RNAs were found to be responsible for the different levels of steady-state mRNA. When either exon 2 or 3 was inserted into a heterologous intron, processing of the primary transcripts generated identical mRNA species while maintaining the differences in exon 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.
Subject(s)
Exons , Gene Expression Regulation, Viral , HIV-1/genetics , HIV-1/metabolism , Alternative Splicing , Cell Line , Cytoplasm/genetics , Gene Products, tat/genetics , Gene Products, tat/metabolism , Genetic Vectors , HIV Long Terminal Repeat , HeLa Cells , Humans , Introns , RNA Stability , RNA, Messenger/genetics , RNA, Messenger/metabolism , RNA, Nuclear , RNA, Viral/genetics , RNA, Viral/metabolism , Transcription, Genetic , Transcriptional Activation , Viral Proteins/genetics , Viral Proteins/metabolism , tat Gene Products, Human Immunodeficiency VirusABSTRACT
HIV-1 env expression from certain subgenomic vectors requires the viral regulatory protein Rev, its target sequence RRE, and a 5' splice site upstream of the env open reading frame. To determine the role of this splice site in the 5'-splice-site-dependent Rev-mediated env gene expression, we have subjected the HIV-1 5' splice site, SD4, to a mutational analysis and have analyzed the effect of those mutations on env expression. The results demonstrate that the overall strength of hydrogen bonding between the 5' splice site, SD4, and the free 5' end of the U1 snRNA correlates with env expression efficiency, as long as env expression is suboptimal, and that a continuous stretch of 14 hydrogen bonds can lead to full env expression, as a result of stabilizing the pre-mRNA. The U1 snRNA-mediated stabilization is independent of functional splicing, as a mismatch in position +1 of the 5' splice site that led to loss of detectable amounts of spliced transcripts did not preclude stabilization and expression of the unspliced env mRNA, provided that Rev enables its nuclear export. The nucleotides capable of participating in U1 snRNA:pre-mRNA interaction include positions -3 to +8 of the 5' splice site and all 11 nt constituting the single-stranded 5' end of U1 snRNA. Moreover, env gene expression is significantly decreased upon the introduction of point mutations in several upstream GAR nucleotide motifs, which are mediating SF2/ASF responsiveness in an in vitro splicing assay. This suggests that the GAR sequences may play a role in stabilizing the pre-mRNA by sequestering U1 snRNP to SD4.
