3 research outputs found
Stretching the genetic code : Incorporation of selenocysteine at specific UGA codons in recombinant proteins produced in Escherichia coli
Selenocysteine (Sec) exists in all domains of life and represents the
21st naturally occurring amino acid. A Sec residue is co-translationally
incorporated at a predefined opal (UGA) codon. UGA codons normally encode
for translational stop via the protein release factor 2 (RF2). The
incorporation mechanism of Sec into the selenoprotein involves complex
machineries, dependent on several specific factors that differ between
organisms. For both eukaryotes and prokaryotes, an mRNA secondary
structure, called a Sec insertion sequence (SECIS) element, is required.
Sec insertion systems for eukaryotes are different from that of bacteria.
Due to the differences between species, recombinant expression of
eukaryotic selenoproteins in E. coli is not a trivial task. However, our
group has previously been able to overcome this species barrier and
successfully expressed the mammalian selenoprotein thioredoxin reductase
1 (TrxR1) and other selenoproteins in E. coli. In this thesis, we have
further developed this recombinant selenoprotein production system. We
have also further characterized the recombinantly expressed rat TrxR1.
We have studied growth conditions affecting yield of the recombinant
selenoprotein when expressing rat TrxR1, using various levels of the
selenoproteinencoding mRNA and growth in different types of medium.
Guided by Principal Component Analysis (PCA), we discovered that the most
efficient bacterial selenoprotein production conditions were obtained
using high-transcription levels in the presence of the selA, selB and
selC genes, with induction of production at late exponential phase. We
also constructed an E. coli strain with the endogenous chromosomal
promoter of the gene for relase factor 2 (RF2), prfB, replaced with the
titrable PBAD promoter. In a turbidostatic fermentor system the
simultaneous impact of prfB knockdown on growth and on recombinant
selenoprotein expression was studied, using rat TrxR1 as the model
selenoprotein. This showed that lower levels of RF2 correlate directly to
an increase of Sec incorporation specificity, while also affecting total
selenoprotein yield concomitant with a slower growth rate.
Recombinant rat TrxR is expressed as a mixture of full-length and
twoamino acid truncated subunits. Phenylarsine oxide (PAO) Sepharose can
be used to enrich the Sec-containing protein. We investigated the
mechanism of this purification by extensively purifying recombinant rat
TrxR1, which gave an enzyme with about 53 U/mg in specific activity,
which was higher than ever reported. Surprisingly, onlyabout 65% of the
subunits of this TrxR1 preparation contained Sec, which revealed a
theoretical maximal specific activity of about 80 U/mg for TrxR with full
Sec content. The high specific activity revealed that the inherent
turnover capacity of rat TrxR1 must be revised, and that the efficiency
of bacterial Sec incorporation may be lower than previously believed. We
also discovered and characterized tetrameric forms of recombinant TrxR1,
having about half the specific activity compared to the dimeric protein
in relation to Sec content.
In conclusion, this thesis describes limiting factors for recombinant
selenoprotein production in E. coli and shows how this production system
can be optimized for higher yield and specificity. The results may prove
to be of importance for the further development of E. coli as a useful
source for synthetic selenoproteins. Results are also presented and
discussed regarding the catalytic capacity of rat TrxR1 and novel
multimeric states of the protein, which could represent unknown
regulatory features of TrxR having potential physiological importance
Assessment of Production Conditions for Efficient Use of Escherichia coli in High-Yield Heterologous Recombinant Selenoprotein Synthesis
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