Translation and Codon Usage in E. coli

When using high-copy expression vectors, which are able to generate up to 100 target protein molecules per cell, the number of copies of lacl repressor provided by the host cell, such as E. coli, is insufficient to insure tight control. Expression yields can be enhanced by supplying extra copies of repressor on a second plasmid. 

High-copy plasmids might also express large amounts of products that are toxic to the host. Without selective pressure and if the product is not toxic, the losses are at low frequency (lO -KT6 per generation) but can increase tremendously when the expressed insert is toxic (Baneyx, 1999). This can even occur under selective pressure because the plasmid population contains a mixture of plasmids with and 

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