The expression host of choice for the expression of many proteins is Escherichia coli as the production of heterologous protein in E. coli is relatively simple and convenient, as well as being rapid and cheap. A large number of E. coli expression plasmids are also available for a wide variety of needs. Other bacteria used for protein production include Bacillus subtilis.
Most heterologous proteins are expressed in the cytoplasm of E. coli. However, not all proteins formed may be soluble in the cytoplasm, and incorrectly folded proteins formed in cytoplasm can form insoluble aggregates called inclusion bodies. Such insoluble proteins will require refolding, which can be an involved process and may not necessarily produce high yield. Proteins which have disulphide bonds are often not able to fold correctly due to the reducing environment in the cytoplasm which prevents such bond formation, and a possible solution is to target the protein to the periplasmic space by the use of an N-terminal signal sequence. Another possibility is to manipulate the redox environment of the cytoplasm. Other more sophisticated systems are also being developed; such systems may allow for the expression of proteins previously thought impossible in E. coli, such as glycosylated proteins.
The promoters used for these vector are usually based on the promoter of the lac operon or the T7 promoter, and they are normally regulated by the lac operator. These promoters may also be hybrids of different promoters, for example, the Tac-Promoter is a hybrid of trp and lac promoters. Note that most commonly used lac or lac-derived promoters are based on the lacUV5 mutant which is insensitive to catabolite repression. This mutant allows for expression of protein under the control of the lac promoter when the growth medium contains glucose since glucose would inhibit gene expression if wild-type lac promoter is used. Presence of glucose nevertheless may still be used to reduce background expression through residual inhibition in some systems.
Examples of E. coli expression vectors are the pGEX series of vectors where glutathione S-transferase is used as a fusion partner and gene expression is under the control of the tac promoter, and the pET series of vectors which uses a T7 promoter.
It is possible to simultaneously express two or more different proteins in E. coli using different plasmids. However, when 2 or more plasmids are used, each plasmid needs to use a different antibiotic selection as well as a different origin of replication, otherwise one of the plasmids may not be stably maintained. Many commonly used plasmids are based on the ColE1 replicon and are therefore incompatible with each other; in order for a ColE1-based plasmid to coexist with another in the same cell, the other would need to be of a different replicon, e.g. a p15A replicon-based plasmid such as the pACYC series of plasmids. Another approach would be to use a single two-cistron vector or design the coding sequences in tandem as a bi- or poly-cistronic construct.