Abstract: The invention relates to an expression vector system based on the regulation of bacterial luminescence (the lux gene system). The invention further relates to the construction of a precisely regulatable expression vector system which comprises a complete luxR gene in combination with an inactivated luxI gene. If the system is turned off, no significant transcription occurs of any cloned gene product when used in combination with the regulatory scheme of the invention as is demonstrated by using the bacteriophage .lambda. lysis genes. The induction of transcription relies on the addition of exogenous autoinducer which is both inexpensive and easy-to-use and which is required in only minute amounts.