In auto-inducing media, lactose serves to induce expression of the T7 RNA polymerase upon conversion to allo-lactose by beta-galactosidase (the lacZ gene product). Therefore, auto-induction should be performed using strains with an intact lac operon (1). In T7 Express strain the T7 RNA polymerase gene (T7 gene1) disrupts the lacZ gene causing inactivation of beta-galactosidase. Therefore T7 Express strain is not recommended for auto-induction procedures.
We recommend host strains BL21(DE3) (NEB #C2527H and NEB #C2527I) or NiCo21(DE3) (NEB #C2529H) for auto-induction procedures.
(1) Studier, F. W. (2005) Protein Production by Auto-Induction in High-Density Shaking Cultures Protein Expr. Purif. 41, 207-34.