The Vibrio fischeri luxICDABEGgenes are upregulated by the autoinducer N-(3-oxohexanoyl) L homoserine lactone and the LuxR protein. The latter contains 250 aa and consists of two domains. The C- terminal domain, which extends from approximately residue 162 to the C end, is thought to bind lux re-gulatory DNA and activate transcription of the luxICDABEG genes. The N-terminal domain, which binds the autoinducer, comprises approximately 70% of LuxR residues. In Escherichia coli, the C-terminal domain can activate lux genes with the absence of the autoinducer. Previously, it was shown that the ATP-dependent Lon protease of E. coli was involved in the downregulation of the V. fischeri lux operon and that LuxR was a target of  

Lon protease. Effects of Lon protease on V. fischeri luxICDABEG gene expression are compared in this study in model plasmids with lux operon genes or DNA fragments encoding either full-length LuxR or its C-terminal domain. It is shown that full-length LuxR, but not C-terminal domain is a target protein for Lon protease. The full-length LuxR protein is a more potent activator than its C-terminal domain. It displays upregulating activity at intracellular concentrations approximately two orders of magnitude lower than its C-terminal domain.