Ribosomal scanning on the 5′-untranslated region of the human immunodeficiency virus RNA genome

Translation initiation on most eukaryotic mRNAs occurs via a cap-dependent scanning mechanism and its efficiency is modulated by their 5′-untranslated regions (5′-UTR). The human immunodeficiency virus type 1 (HIV-1) 5′-UTR contains a stable TAR hairpin directly at its 5′-end, which possibly masks the cap structure. In addition, the 5′-UTR is relatively long and contains several stable RNA structures that are essential for viral replication. These characteristics may interfere with ribosomal scanning and suggest that translation is initiated via internal entry of ribosomes. Literature on the HIV-1 5′-UTR-driven translation initiation mechanism is controversial. Both scanning and internal initiation have been shown to occur in various experimental systems. To gain further insight in the translation initiation process, we determined which part of the 5′-UTR is scanned. To do so, we introduced upstream AUGs at various positions across the 5′-UTR and determined the effect on expression of a downstream reporter gene that was placed under control of the gag start codon. This strategy allowed us to determine the window of ribosomal scanning on the HIV-1 5′-UTR.