Shuttle vector systems are provided for the regulated expression of subject genes in transformed hosts. A promoter/operator which does not originate with the transformed host cell controls expression of the subject gene and a repressor, which also does not originate with the host cell, regulates the promoter/operator. According to this invention, promoter/operator--repressor functionalities from one bacterium are used in other bacteria, thereby obviating the need to locate homologous expression regulatory sequences for each bacterial host.