Expression systems protein-tagging sequence

The E. coli expression system used is based on the pQE-80L expression system. Sequences inserted into the multiple cloning site can be expressed as native proteins bearing an N-terminal hexahistidine tag upon IPTG induction of the T5 promoter in suitably transformed E. coli cells. 

A key constraint in characterizing the products of these cDNAs is that often these sequences have no known function. Therefore, in expressing these cDNAs as recombinant proteins, one does not have an assay for activity to follow purification. Consequently, efforts to express these recombinant proteins rely on fusion constructs, multiple expression systems, and purification schemes that depend entirely on the presence of affinity tags. Expression and purification methods are designed to be independent of the characteristics of the protein of interest. 

Over several decades, multiple vector systems for recombinant gene expression in E. coli have been developed. Modem vectors suitable for recombinant protein production vary in the used promoter system in the presence or absence of coding sequences for affinity tags upstream or downstream of the multiple cloning site (MCS) and of sequences coding for leader peptides for the protein export. Moreover, different origins of replication (ori), antibiotic selection marker genes and MCS are used. 

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