Assessment of Production Conditions for Efficient Use ofEscherichia coliin High-Yield Heterologous Recombinant Selenoprotein Synthesis

Abstract

The production of heterologous selenoproteins in Escherichia coli necessitates the design of a secondary structure in the mRNA forming a selenocysteine insertion sequence (SECIS) element compatible with SelB, the elongation factor for selenocysteine insertion at a predefined UGA codon. SelB competes with release factor 2 (RF2) catalyzing translational termination at UGA. Stoichiometry between mRNA, the SelB elongation factor, and RF2 is thereby important, whereas other expression conditions affecting the yield of recombinant selenoproteins have been poorly assessed. Here we expressed the rat selenoprotein thioredoxin reductase, with titrated levels of the selenoprotein mRNA under diverse growth conditions, with or without cotransformation of the accessory bacterial selA, selB, and selC genes. Titration of the selenoprotein mRNA with a pBAD promoter was performed in both TOP10 and BW27783 cells, which unexpectedly could not improve yield or specific activity compared to that achieved in our prior studies. Guided by principal component analysis, we instead discovered that the most efficient bacterial selenoprotein production conditions were obtained with the high-transcription T7lac-driven pET vector system in presence of the selA, selB, and selC genes, with induction of production at late exponential phase. About 40 mg of rat thioredoxin reductase with 50% selenocysteine content could thereby be produced per liter bacterial culture. These findings clearly illustrate the ability of E. coli to upregulate the selenocysteine incorporation machinery on demand and that this is furthermore strongly augmented in late exponential phase. This study also demonstrates that E. coli can indeed be utilized as cell factories for highly efficient production of heterologous selenoproteins such as rat thioredoxin reductase.Many organisms express selenoproteins, carrying a selenocysteine residue, the 21st naturally occurring amino acid (7,16,26,34). Selenocysteine is cotranslationally inserted at the position of an opal (UGA) codon, which normally confers termination of translation. The UGA codon is recoded as selenocysteine by complex translation machineries that differ between gram-negative (7, 16) and gram-positive bacteria (13), archaea (31), and higher eukaryotes (9,12,30). The translation system in Escherichia coli is the most characterized (reviewed in references 7, 16, and 34). Briefly, the mRNA for an E. coli selenoprotein carries a specific sequence after the UGA codon, both encoding the amino acids following the selenocysteine residue and forming a stem-loop secondary structure, a so-called selenocysteine insertion sequence (SECIS) element.The SECIS element binds the SelB elongation factor, the selB gene product. SelB is homologous to elongation factor Tu (EF-Tu) but, in addition, binds the loop of the SECIS element through an additional C-terminal domain. In terms of tRNA substrate, SelB is only functional with the selenocysteine-specific tRNA Sec , the selC gene product, in its selenocysteinylated form. By analogy with ...

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“…Wild-type human Trx1, human Grx1, Escherichia coli Grx1 C14S mutant protein, and anti-human Trx1 antibody were products of IMCO Ltd. (Stockholm, Sweden). Recombinant TrxR1 was a gift from Dr. Elias Arnér (Department of Medical Biochemistry and Biophysics, Karolinska Institutet) and was purified as described (21).…”

Multilevel Regulation of 2-Cys Peroxiredoxin Reaction Cycle by S-Nitrosylation

“…Sang Won Kang (Ewha Womans University). Recombinant rat TrxR1 was produced as previously described (24) and had specific activity of 28 units/mg. Other materials were obtained from Sigma unless otherwise indicated.…”

Crystal Structure and Catalysis of the Selenoprotein Thioredoxin Reductase 1

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