Has reversible, light-dependent DNA-binding activity. Upon illumination an internal FMN-protein adduct is formed which changes the protein conformation so that the previously sequestered DNA-binding domain is free to bind DNA. Binds to sequences within in its own promoter when illuminated but not when it has been incubated in the dark. Monomer in solution, probably binds DNA as a homodimer. Light induces structural changes; in the dark protein is compact and resistant to exogenous protease, in the light protein is less compact while protease treatment generates an N-terminal fragment corresponding to approximately residues 14-156 (the LOV domain). FMN binds covalently to cysteine after exposure to blue light, this bond is spontaneously broken in the dark; Cys-78 is 3.9 Angstroms from C4a of FMN in the dark, suggesting this is the adduct that is made. It is uncertain whether Met-1 or Met-4 is the initiator.