BVDV-RNA translation in mammalian cells is mediated by a cap-independent, internal initiation of translation. Left panel: Schematic representation of the mRNA arising from the expression plasmids stably transfected into BHK cells. Triangle, cap structure; solid line, intercistronic region with 5' UTRs of BVDV strain NADL, BVDV strain SD-1, or poliovirus type I. White box, SEAP reporter gene (secreted form of the alkaline phosphatase human placenta); grey box, luciferase reporter gene. The stem-loop structure in construct 3 has a calculated stability of about -73 kcal/mol. Right panel: Relative SEAP and luciferase expression values, levelled out to the specific mRNA content after Northern Blot quantification using a phosphorimager (see Materials and Methods).