T7 Expression

Within the realm of E. coli expression, the T7 system is the most popular approach for producing recombinant protein. In this system, the target gene is cloned into an expression vector downstream of the T7 promoter and this construct is introduced into a T7 expression host. T7 expression hosts such as DE3 prophage strains or T7 Express strains carry a chromosomal copy of the phage T7 RNA polymerase gene. When inducer is added, T7 RNA polymerase is expressed and becomes dedicated to target gene transcription. T7 expression is often very robust but may suffer from undesirable basal (non-induced) transcription especially in DE3 strains. Regulation of IPTG-induced T7 expression is accomplished by co-expressing the lac repressor from a plasmid or a host-encoded lacI gene and by co-expressing T7 lysozyme, the natural inhibitor of T7 RNA polymerase activity. T7 lysozyme may be expressed from pLysS or pLysE plasmids or a variant T7 lysozyme may be expressed from the lysY gene present within multiple NEB protein expression strains. The lysY gene product lacks amidase (lysozyme) activity against the E. coli cell wall while retaining the ability to inhibit basal T7 RNA polymerase activity.