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 lysY/Iq strain the T7 RNA polymerase gene (T7 gene1) disrupts the lacZ gene causing inactivation of beta-galactosidase. Therefore T7 Express lysY/Iq 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.