Three-dimensional crystal structure of the transcription factor PhoB receiver domain

PhoB is the response regulator of the two-component signal transduction sys tem activated under phosphate starvation conditions. This protein is a tran scription factor that activates more than 30 genes of the pho regulon and c onsists of two domains: a DNA binding domain and a dimerization domain, the latter being homologous to the receiver domain described for two-component response regulators. Activation by phosphorylation induces dimerization of the protein and the consequent binding to the DNA direct repeat pho box, w here it promotes the binding of RNA polymerase. In the absence of phosphory lation, the activating dimerization process can be mimicked by deletion of the DNA binding domain. The three-dimensional crystal structure of the rece iver domain of PhoB from Escherichia coil has been solved by multiple anoma lous diffraction using a gold derivative obtained by co-crystallization, an d refined using data to 1.9 Angstrom resolution. The crystal structure reve als an alpha/beta doubly wound fold, similar to other known receivers, the most conspicuous difference being the displacement of helix alpha 4 towards its N terminus. The active site includes the acidic triad Asp53 (the site of phosphorylation), Asp10 and Glu9. Lys105, from loop beta 5 alpha 5, and Glu88, from helix alpha 4, interact with Asp53 via an H-bond and a water br idge, respectively. Ln the asymmetric unit of the crystal there are two mol ecules linked by a complementary hydrophobic surface, which involves helix alpha 1, loop beta 5 alpha 5 and the N terminus of helix alpha 5, and is co nnected to the active site through the fully conserved residue Lys105 from loop beta 5 alpha 5. The possibility that this surface is the functional su rface used for the activating dimerization is discussed. (C) 1999 Academic Press.