Explain how engineered E.coli expression strains like Codon-Plus and Rosetta overcome codon bias.

Engineered E.coli expression strains like Codon-Plus and Rosetta overcome codon bias by introducing tRNAs that recognize and translate rare codons. Codon bias refers to the uneven usage of codons in a coding sequence due to differences in codon frequencies and the availability of tRNAs. The rare codons become a bottleneck in the protein expression process as they are not efficiently translated by the host's tRNAs. The introduction of new tRNAs targeted to these rare codons increases the availability of the required tRNAs and reduces the pressure on the protein translation machinery. This improves the efficiency of protein expression and yields higher expression levels of the desired protein.