H-NS controls the transcription of three promoters of Vibrio fischeri lux cloned in Escherichia coli.

We have recently proposed that the expression of V. fischeri right lux operon is controlled by two promoters; the first one located upstream of the luxl gene, while the second one seems to be located upstream of the luxC gene. The transcription from both promoters is negatively controlled by H-NS protein. Escherichia coli MC4100 rpoS hns mutant that carried the V. fischeri lux system with a deletion in either the luxl or luxR gene showed a constitutive mode and more than 10,000-fold higher luminescence than the control cells. The present study shows that neither luxR nor luxl are required for the transcription of the luxCDABE genes in an H-NS deficient strain of E. coli. The MC4100 rpoS hns mutant harbouring the luxCDABE-carrying plasmid showed constitutive mode and 70,000-fold higher luminescence than the wild-type cells. The question whether both the left and the right operons of V. fischeri lux system are controlled by H-NS was addressed with the aid of plasmids harbouring the lacZ gene fused with luxR or luxl. In MC4100 hns rpoS background, luxR and luxl genes were very early and actively transcribed, as judged by the strong beta-galactosidase activity that was developed at early stage of growth. The beta-galactosidase activity in the wild-type cells was 20-40 times lower and occurred mainly during the second half of the growth cycle. It thus appears that H-NS inhibits the transcription of three promoters of the lux system of V. fischeri; the left operon that codes for LuxR protein and two promoters located upstream and downstream to luxl gene.