Expressing recombinant selenoproteins using redefinition of a single UAG codon in an RF1-depleted E. coli host strain

Selenoproteins containing the rare amino acid selenocysteine (Sec), typically being enzymes utilizing the selenium atom of Sec for promoted catalysis of redox reactions, are challenging to obtain at high amounts in pure form. The technical challenges limiting selenoprotein supply derive from intricacies in their translation, necessitating the recoding of a UGA stop codon to a sense codon for Sec. This, in turn, involves the interactions of a Sec-dedicated elongation factor, either directly or indirectly, with a structure in the selenoprotein-encoding mRNA called a SECIS element (Selenocysteine Insertion Sequence), a dedicated tRNA species for Sec with an anticodon for the UGA, and several accessory enzymes and proteins involved in the selenoprotein synthesis. Here, we describe an alternative method for recombinant selenoprotein production using UAG as the Sec codon in a specific strain of E. coli lacking other UAG codons and lacking the release factor RF1 that normally terminates translation at UAG. We also describe how such recombinant selenoproteins can be purified and further analyzed for final Sec contents. The methodology can be used for production of natural selenoproteins in recombinant form as well as for production of synthetic selenoproteins that may be designed to use the unique biophysical properties of Sec for diverse biotechnological applications.