Retroviruses as Model Systems for the Study of Nuclear RNA Export Pathways

Retroviral replication requires the nuclear export and cytoplasmic translation of both incompletely spliced and fully spliced forms of the initial, genome length viral transcript. Thus in most simple retroviruses, this initial transcript leaves the nucleus as both an unspliced mRNA encoding Gag and Gag-Pol and as a singly spliced mRNA encoding Env. More complex retroviruses derive a range of unspliced, incompletely spliced and multiply spliced mRNAs from their single initial RNA transcript. For example, human immunodeficiency virus type 1 (HIV-1), which remains the most complex retrovirus known, encodes 1 unspliced similar to 9-kb RNA encoding Gag and Gag-Pol, 5 singly spliced, similar to 4-kb mRNAs encoding Vif, Vpr, Vpu, and Env, and finally similar to 16 multiply spliced similar to 2-kb mRNAs encoding Tat, Rev, and Nef.