Figure 5.HIV-1 IRES containing transcripts employ a “land and scan” mechanism. A. IRES activity was measured from cells transfected with dual-luciferase DNA constructs containing WT leader sequences and treated with increasing concentrations of the eIF4A inhibitor hippuristanol. B. An uAUG was inserted out of frame from the FLUC AUG in each leader specific sequence. C. IRES activity was measured from Jurkat T-cells transfected with dual-luciferase DNA constructs containing WT and uAUG sequences (as shown in panel B). IRES activity from each WT leader construct is set at 100%. D. Diagram of the “land and scan” model for initiation from different HIV-1 transcripts. In this model, the common exon structure, likely bound by ITAFs (blue and orange ovals), serves as the site for entry of the 40S subunit and associated factors (yellow, labeled 40S(+)). The 40S(+) scans through each LSS to reach the start codon. Different lengths, sequences, structures, bound proteins, etc. in the different LSSs could modulate the efficiency of scanning and thus of translation initiation.