NAD-dependent DNA-binding activity of the bifunctional NadR regulator of Salmonella typhimurium

NadR is a 45-kDa bifunctional regulator protein. In vivo genetic studies indicate that NadR represses three genes involved in the biosynthesis of NAD. It also participates with an integral membrane protein (PnuC) in the import of nicotinamide mononucleotide, an NAD precursor. NadR was overexpressed and purified as a His-tagged fusion in order to study its DNA-binding properties. The protein bound to DNA fragments containing NAD box consensus sequences. NAD proved to be the relevant in vivo corepressor, but full NAD dependence of repressor activity required nucleotide triphosphates. DNA footprint analysis and gel shift assays suggest that NadR binds as a multimer to adjacent NAD boxes. The DNA-repressor complex would sequester a potential RNA polymerase binding site and thereby decrease expression of the nad regulon.