THE SIMIAN RETROVIRUS-1 CONSTITUTIVE TRANSPORT ELEMENT, UNLIKE THE HIV-1 RRE, USES FACTORS REQUIRED FOR CELLULAR MESSENGER-RNA EXPORT

Background: A hallmark of retroviral gene expression is that unspliced retroviral genomic RNA is exported to the cytoplasm, whereas endogeno us intron containing cellular RNAs are usually retained in the nucleus . In complex retroviruses, such as human immunodeficiency virus-1 (HIV -1), nuclear export is accomplished by the interaction of a virally en coded protein, Rev, with a cis-acting RNA element, the Rev-responsive element (RRE), In type D retroviruses, such as the simian retrovirus t ype 1 (SRV-1), however, genomic RNA is exported by cellular factor(s) that interact with a conserved cis-acting RNA element, the constitutiv e transport element (CTE), Results: We found that the CTE was exported in a specific and saturable fashion from Xenopus oocyte nuclei, When inserted into the intron of an adenovirus-derived pre-mRNA, the CTE di d not affect splicing efficiency but promoted the nuclear export of th e excised intron lariat that is normally retained within the nucleus, Export of CTE-containing RNAs to the cytoplasm was not affected by the heterogeneous nuclear ribonucleoprotein Al or an excess of peptides c orresponding to the Rev nuclear export signal, Microinjection of satur ating amounts of CTE RNA did not affect tRNA export or Rev-mediated ex port but did inhibit mRNA export, CTE-mediated export was found to be dependent on Ran-mediated GTP hydrolysis. Conclusion: The Rev-RRE syst em and the CTE direct intron-containing RNAs to distinct export pathwa ys, Although previous data have suggested that Rev uses the same expor t pathway as uracil-rich small nuclear RNAs and 5S ribosomal RNA, the CTE seems to interact with evolutionarily conserved factors that are e ssential for cellular mRNA export.