The present invention provides an improved prokaryotic cell expression system employing a tightly controlled host strain construct that controls uninduced, leaky expression of proteins while still auto-inducing well. Various aspects of the present invention address and overcome the problem of uninduced basal expression by providing a host strain that comprises a T7 polymerase gene, and mutants thereof, inserted between lac Z and lac Y of the lac operon (a “ZRY” construct), downstream of an otherwise wild-type lac operon control region.