The influence of BVDV coding sequences on IRES mediated translation. Left panel: Schematic drawing of dicistronic (1–3) and monocistronic (4–6) plasmids carrying the complete BVDV-5' UTR or the 5' UTR plus 5' proximal BVDV SD-1 coding regions. Right panel: Relative translation efficiency of SEAP and luciferase reporter genes of the respective dicistronic or monocistronic plasmids in stably transfected BHK cells. The SEAP and luciferase values are normalized to specific mRNA levels. Luc (Eff): Luciferase values exhibited by the moncistronic constructs were taken into account to calculate the effect of the inclusion of coding region on IRES mediated translation. The AUG context at position +4 (G) and +5 (A) of the wild-type luciferase construct and its fusion mutants is identical and optimal and should not give rise altered translational efficiency . The data shown are average values derived from four independent experiments. Addition of 9 or 25 codons of the BVDV aminoterminus (black box) to the SD-1 5' UTR results in 3 to 4.8 fold increase of translation efficiency when the effects of N-terminal extension of luciferase in monocistronic constructs on luciferase stability/activity were considered.