BglG, the response regulator of the Escherichia coli bgl operon, is phosphorylated on a histidine residue

We have shown previously that the activity of BglG, the response regulator of the bgl system, as a transcriptional antiterminator is modulated by the sensor BgIF, which reversibly phosphorylates BglG. We show here that the phosphoryl group on BglG is present as a phosphoramidate, based on the sensitivity of phosphorylated BglG to heat, hydroxylamine, and acidic but not basic conditions. By analyzing the products of base-hydrolyzed phosphorylated BglG by thin-layer chromatography, we show that the phosphorylation occurs on a histidine residue. This result supports the notion that the bgl system is a member of a new family of bacterial sensory systems.