| Rous sarcoma virus (RSV), like all retroviruses, must produce both spliced and unspliced RNA from the same primary transcript and export them to the cytoplasm for viral replication. Two cis-acting RNA elements within the genome of RSV are responsible for the maintenance and export of the unspliced viral RNA specie, the negative regulator of splicing (NRS), and the direct repeats (DR). The NRS element binds U1 snRNP at a sequence that deviates from the 5' splice site consensus by a substitution of U's for A's at three positions: -2, +3 and +4. We have found that all three of these U's are important for NRS activity. Mutagenesis of non-consensus U's can decrease NRS activity or activate cryptic splicing, depending on the base substitution. This suggests that the NRS is also involved in the repression of cryptic splicing, as well as the maintenance of unspliced RNA. The cytoplasmic accumulation of full-length viral RNA is promoted by two cis-acting direct repeat (DR) elements that flank the src gene. Unlike simple retoviruses, complex retroviruses encode auxiliary proteins that are responsible for export of their partially spliced and unspliced RNA. However, RSV, a simple retrovirus, must depend solely on cellular factors for RNA export. I show here that the DR mediates export of a reporter construct from the nucleus, suggesting it is a constitutive transport element (CTE). I also show that DR-mediated export is CRM-1 independent, suggesting it uses a different pathway than that of complex retroviruses. The DR was also unable to interact with cellular export protein Tap, which interacts with the simple simian retrovirus CTE for RNA export. These data suggest that the RSV DR element uses a novel nucleocytoplasmic export pathway. |
