Influence of Rex and intronic sequences on expression of spliced mRNAs produced by human T cell leukemia virus type I

Expression of the incompletely spliced HTLV-I mRNAs relies on the viral pos ttranscriptional activator Rex, whose interaction with the Rex-responsive e lement (RXRE) overcomes effects of cis-acting repressive sequences (CRSs), Studies based on heterologous reporter plasmids identified an intronic CRS in the 5' LTR and a CRS that overlaps with the RXRE, The present study inve stigated the effects of these elements in the context of spliced viral mRNA s encoding p21Rex (mRNA 1-3), Tax/Rex (mRNA 1-2-3), and Tof (mRNA 1-2-B), A ll three mRNAs were inefficiently expressed when transcribed in their matur e intronless form, with the p21Rex mRNA showing the weakest expression. In contrast, efficient expression of p21Rex was obtained from a plasmid contai ning the 5' LTR and 3' portion of the genome that encoded a spliceable RNA. The defective expression of the intronless mRNAs reflected the inhibitory activity of the RXRE and the lack of 5' intronic sequences. Insertion of an intronic 5' LTR segment located upstream of the 5' CRS overcame Rex depend ence conferred by the RXRE, The activity of this segment was mapped to the major splice donor and sequences overlapping with, but functionally distinc t from, a previously described transcriptional enhancer. The three mRNAs re sponded differently to Rex and to insertion of the constitutive transport e lement of simian retrovirus type 1, Taken together, these results suggest t hat expression of the spliced mRNAs is controlled by the relative influence of positive and negative sequences present on the primary transcript as we ll as the Rex-RXRE interaction.