A mini-IRES sequence for stringent selection of high producers.

Internal Ribosome Entry Site (IRES) sequences have been widely used to link the expression of two independent proteins on the same mRNA transcript. Genes encoding fluorescent proteins or drug-resistance enzymes are usually placed downstream of IRES, serving as expression indicators or selection markers. In biological applications where the upstream gene-of-interest is to be expressed at extremely high levels, it is often desirable to purposely reduce IRES downstream gene expression to economize the cellular resources and/or to generate more stringent selection pressure. Here we describe a miniature IRES mutant sequence (IRESmut3) with dramatically diminished co-translational efficiency to fulfill these purposes.