Involved in the regulation of different processes depending on the cell density. Acts together with sigma-54 to repress, perhaps indirectly, some genes. In strain El Tor C6706, at low cell density, LuxO acts indirectly on virulence gene expression by repressing hapR This leads to the expression of virulence factors. In strain El Tor N16961, the hapR gene is inactive due to a natural frameshift mutation.