Escherichia coli is a well-established and popular host for heterologous expression
of proteins. The preference in the choice of synonymous codons (codon bias), however,
might differ for the host and the original source of the recombinant protein, constituting
a potential bottleneck in production. Codon choice affects the efficiency of translation
by a complex and poorly understood mechanism. The availability of certain tRNA species
is one of the factors that may curtail the capacity of translation. Here we provide
a tRNA-overexpressing strategy that allows the resolution of the codon bias, and boosts
the translational capacity of the popular host BL21(DE3) when rare codons are encountered.
In the BL21(DE3)-derived strain, called SixPack, copies of the genes corresponding
to the six least abundant tRNA species have been assembled in a synthetic fragment
and inserted into a rRNA operon. This arrangement, while not interfering with the
growth properties of the new strain, allows dynamic control of the transcription of
the extra tRNA genes, providing significantly elevated levels of the rare tRNAs in
the exponential growth phase. Results from expression assays of a panel of recombinant
proteins of diverse origin and codon composition showed that the performance of SixPack
surpassed that of the parental BL21(DE3) or a related strain equipped with a rare
tRNA-expressing plasmid.
