Upstream AUG Codons in the Simian Immunodeficiency Virus SIVmac239 Genome Regulate Rev and Env Protein Translation

ThemRNAs encoding the Rev and Env proteins of simian immunodeficiency virus (SIV) are unique because upstream transla-tion start codons are present that maymodulate the expression of these viral proteins. This is true for the regular mRNAs, but we also report novel mRNA splicing variants that encode up to five upstream AUG (uAUG) codons. Their influence on Rev and Env translation was measured by mutational inactivation in reporter constructs and in the SIVmac239 strain. An intricate regulatory mechanism was disclosed that allows the virus to express a balanced amount of these two proteins. This insight also allows the design of vector constructs that efficiently express these proteins. The RNA genome of the simian immunodeficiency virus (SIV)displays a complex splicing pattern with several splice donor (SD) and splice acceptor (SA) sites (Fig. 1) to allow the expression of all essential, regulatory, and accessory proteins (45, 51, 57). Translation initiation on eukaryotic mRNAs generally occurs via ribosomal scanning (34), in which translation initiation factors interact with the 40S ribosomal subunit at themRNA5 = cap struc-ture. The ribosomal subunit then scans along the 5 = untranslated region (UTR) until it encounters an AUG translational start codon in a favorable sequence context. The optimal context of a