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For proteins that do not need to be synthesized in a glycosylated or extensively post-translational modification, bacteria are an excellent expression host due to their relatively simplicity in terms of biology and process. Bacterial expression tends to be less expensive than eukaryotic cell expression because of lower medium costs and shorter processing times. The expression of recombinant proteins requires the selection of an appropriate cloning vector, expression elements and a host for clone generation, the construction of stable, high-yield and high-quality recombinant clone, a scalable, consistent and optimized fermentation processes, and a cost-effective and convenient purification process. The most commonly used bacterium for recombinant protein production is Escherichia coli (E. coli), an intestinal bacterium with a long history of safe use in the laboratory and industry. E. coli is a particularly suitable host because it is well characterized physiologically and metabolically, its entire genome has been sequenced, and many molecular biology tools are available to engineer its DNA sequence to generate novel functionality. The gene encoding the desired protein is first cloned into the multiple cloning site (MCS) of the expression vector, which is under the control of a promoter that regulates gene expression. The vectors are transformed into E. coli, and the transformants are grown in liquid culture. At specific growth stages, recombinant protein production is induced by the addition of a chemical inducer that will activate the promoter on the expression vector. Thus, the recombinant gene is expressed and the recombinant polypeptide chain folds into the recombinant protein of interest. The recombinant proteins can then be released, captured, and purified.

The choice of expression vector is a key driver for the successful expression of recombinant proteins in bacterial. Expression vectors consist of a plasmid backbone that contains genetic elements, including an origin of replication, a selection marker, a promoter driving the coding sequence expression, a ribosome binding site, a terminator for the coding sequence, and other genetic elements. A promoter is usually regulated by chemical inducer that activates transcription when added to the culture. Thus, the temporal control of protein production and separation of the host growth and protein production phases of the process can be achieved. An ideal promoter is completely "off" in the absence of the inducer, and adjustable expression level depends on the concentration of inducer molecules added to the culture. Commonly used promoters include T7 promoter, lac promoter, araBAD promoter, and T5 promoter. When designing expression vectors, the selection of elements is important for the characteristics of the expression vectors, particularly the promoter that initiates the transcription and drives the expression of gene of interest, the selection marker that enables maintenance of the plasmid in the host organism, and the origin of replication that controls the plasmid copy number. Apart from the basic components of the expression vector, the expression vector can be modified with tags to enhance recombinant protein productivity and folding. The nucleotide sequences encoding these tags are fused directly to the recombinant gene that is being synthesized, at either the N or C terminus.