A balanced constitutive inducible transcriptional control system is provided to facilitate the expression of polypeptides incompatible with recombinant host cells. Use of this system has, for the first time, made it possible to produce mature, unglycosylated human tissue plasminogen activator in prokaryotes which is water soluble and fully enzymatically active. In accordance with this invention, t-PA is produced by recombinant bacterial host cell culture in commercially significant amounts without in vitro renaturation and processing.