Mapping of internal ribosome entry sites (IRES)

The dicistronic luciferase reporter gene system is the most common method to isolate and characterize internal ribosome entry sites (IRES). It is based on the expression of a dicistronic RNA comprising two independent reporter genes in 3' and 5'cistrons, and the putative IRES inserted into intercistronic region. The most convenient aspect of using Renilla and firefly luciferase genes is that both gene products can be detected in a single assay using Dual-Luciferase(®) Reporter Assay System from Promega. The Renilla luciferase coding sequence is often inserted into the 5'cistron and serves as internal control. It is translated cap dependently, as it is close to the cap structure at the 5' end. The 3'cistron located far downstream to the cap structure can only be translated by a cap-independent mechanism when the intercistronic sequence is capable of ribosome binding and re-initiation of translation. Expression level of the 3'cistron is usually normalized to the expression of 5'cistron to estimate the relative IRES activity of intercistronic sequences.