Acts negatively to control the expression of luminescence. At low cell density, LuxO is phosphorylated, and together with sigma-54, causes repression of the luxCDABEGH operon. This repression could be indirect, LuxO could activate a negative regulator of luminescence. At high cell density, LuxO is dephosphorylated and inactive, therefore the luxCDABEGH operon is not repressed and light is emitted. LuxO and sigma-54 have also a role in activating the production of siderophore and in regulating the rugose colony morphology phenotype (By similarity).