AUTOINDUCER-INDEPENDENT MUTANTS OF THE LUXR TRANSCRIPTIONAL ACTIVATOREXHIBIT DIFFERENTIAL-EFFECTS ON THE 2 LUX PROMOTERS OF VIBRIO-FISCHERI

The LuxR protein is a transcriptional activator which, together with a diffusible small molecule termed the autoinducer [N-(3-oxohexanoyl)-L -homoserine lactone], represents the primary level of regulation of th e bioluminescence genes in Vibrio fischeri. LuxR, in the presence of a utoinducer, activates transcription of the luxICDABEG gene cluster and both positively and negatively autoregulates transcription of the div ergently oriented luxR gene, activating transcription at low levels of autoinducer, and repressing synthesis at high autoinducer concentrati on. Seven LuxR point mutants which activate V. fischeri lux transcript ion in the absence of autoinducer (LuxR) have been characterized. The LuxR proteins activated transcription of the bioluminescence genes t o levels 1.5-40 times that achieved by wild-type LuxR without autoindu cer. All of the LuxR mutants retained responsiveness to autoinducer. However, in each case the degree of stimulation in response to autoind ucer was lower than that observed for wild-type LuxR. The LuxR protei ns retained the requirement for autoinducer for autoregulation of the luxR gene. We propose that the LuxR protein exists in two conformation s, an inactive form, and an active form which predominates in the pres ence of autoinducer. The LuxR mutations appear to shift the equilibri um distribution of these two forms so as to increase the amount of the active form in the absence of autoinducer, while autoinducer can stil l convert inactive to active species. The differential effects of the LuxR proteins at the two lux promoters suggest that LuxR stimulates P -luxR transcription by a different mechanism to that used at the P-lux I promoter, implying that binding of LuxR to its binding site, known t o be necessary for transcriptional activation, may not be sufficient.