Bacterial expression systems, primarily Escherichia coli (E. coli) offer the advantage of obtaining large amounts of recombinant protein in a relatively short period time and at low cost. On a routine basis the pET expression system is being used. A series of expression vectors based on the backbone of the pET vectors (Novagen) were constructed to contain the tobacco etch virus (TEV) protease recognition site.
In the case of multi-complex protein expression, the target genes can be introduced by co-transformation of compatible plasmids or using a single plasmid carrying multiple target genes under the same promoter or separate promoters. Proteins can be expressed in the cytoplasmic fraction, targeted to the periplasm or as inclusion bodies.
The biggest obstacle in protein expression in E. coli, especially for eukaryotic proteins lies in obtaining large amounts of soluble proteins, corrected folded and active.