The LytTR domain is a DNA-binding motif found within the AlgR/AgrA/LytR family of transcription factors that regulate virulence factor and toxin gene expression in pathogenic bacteria. This previously uncharacterized domain lacks sequence similarity with proteins of known structure. The crystal structure of the DNA-binding domain of Staphylococcus aureus AgrA complexed with a DNA pentadecamer duplex has been determined at 1.6 A resolution. The structure establishes a 10-stranded beta fold for the LytTR domain and reveals its mode of interaction with DNA. Residues within loop regions of AgrA contact two successive major grooves and the intervening minor groove on one face of the oligonucleotide duplex, inducing a substantial bend in the DNA. Loss of DNA binding upon substitution of key interacting residues in AgrA supports the observed binding mode. This mode of protein-DNA interaction provides a potential target for future antimicrobial drug design.

The N-terminal regulatory domains of bacterial response regulator proteins catalyze phosphoryl transfer and function as phosphorylation-dependent regulatory switches to control the output activities of C-terminal effector domains. Structures of numerous isolated regulatory and effector domains have been determined. However, a detailed understanding of regulatory interactions among these domains has been limited by the relative paucity of structural data for intact multidomain response regulator proteins. The first multidomain structures determined, those of transcription factor NarL and methylesterase CheB, both revealed extensive interdomain interfaces. The regulatory domains obstruct access to the functional sites of the effector domains, indicating a regulatory mechanism based on inhibition. In contrast, the recently determined structure of the OmpR/PhoB homologue DrrD revealed no significant interdomain interface, suggesting that the domains are tethered by a flexible linker and lack a fixed orientation relative to each other. To address the generality of this feature, we have determined the 1.8-Å resolution crystal structure of Thermotoga maritima DrrB, providing a second structure of a multidomain response regulator of the OmpR/PhoB subfamily. The structure reveals an extensive domain interface of 751 Å 2 and therefore differs greatly from that observed in DrrD. Residues that are crucial players in defining the activation state of the regulatory domain contribute to this interface, implying that conformational changes associated with phosphorylation will influence these intramolecular contacts. The DrrB and DrrD structures are suggestive of different signaling mechanisms, with intramolecular communication between N-and C-terminal domains making substantially different contributions to effector domain regulation in individual members of the OmpR/PhoB family.The use of modular phosphorylation-regulated switch domains to control effector protein activity is a prevalent theme in both eukaryotic and prokaryotic signal transduction pathways. In eukaryotes, the modular switch is often a small GTPase of the ras superfamily, in which a single phosphate moiety of a bound guanine nucleotide (GTP versus GDP) dictates the activation state of the protein and its intermolecular associations with downstream targets (for a review, see reference 43). In prokaryotes, the switch is typically the regulatory domain of a multidomain response regulator protein that is paired with a histidine protein kinase within a twocomponent signaling system (for reviews, see references 34 and 45). Phosphorylation of a specific aspartyl residue in the regulatory domain of the response regulator protein results in activation of an attached effector domain and generation of the output response of the signaling pathway. In both families of switch proteins, phosphorylation-induced conformational changes alter molecular surfaces that are exploited for proteinprotein interactions specific to the unphosphorylated or phosphorylated states. This provid...

Bacterial response regulators are key regulatory proteins that function as the final elements of socalled two-component signaling systems. The activities of response regulators in vivo are modulated by phosphorylation that results from interactions between the response regulator and its cognate histidine protein kinase. The level of response regulator phosphorylation, which is regulated by intraor extracellular signals sensed by the histidine protein kinase, ultimately determines the output response that is initiated or carried out by the response regulator. We have recently hypothesized that in the OmpR/PhoB subfamily of response regulator transcription factors, this activation involves a common mechanism of dimerization using a set of highly conserved residues in the a4-b5-a5 face. Here we report the X-ray crystal structures of the regulatory domains of response regulators TorR (1.8 Å ), Ca 2+ -bound KdpE (2.0 Å ), and Mg 2+ /BeF 3 --bound KdpE (2.2 Å ), both members of the OmpR/ PhoB subfamily from Escherichia coli. Both regulatory domains form symmetric dimers in the asymmetric unit that involve the a4-b5-a5 face. As observed previously in other OmpR/PhoB response regulators, the dimer interfaces are mediated by highly conserved residues within this subfamily. These results provide further evidence that most all response regulators of the OmpR/ PhoB subfamily share a common mechanism of activation by dimerization.

The OmpR/PhoB family of response regulators (RRs) is the largest class of two-component system (TCS) signal transduction proteins. Extensive biochemical and structural characterization of these transcription factors has provided insights into their activation and DNA-binding mechanisms. For the most part, OmpR/PhoB family proteins are thought to become activated through phosphorylation from their cognate histidine kinase (HK) partners, which in turn facilitates an allosteric change in the RR enabling homodimerization and subsequently enhanced DNA binding. Incongruently, it has been suggested that OmpR, the eponymous member of this RR family, becomes activated via different mechanisms, whereby DNA binding plays a central role in facilitating dimerization and phosphorylation. Characterization of the rate and extent of the phosphorylation of OmpR and OmpR DNA-binding mutants following activation of the EnvZ/OmpR TCS shows that DNA binding is not essential for phosphorylation of OmpR in vivo. In addition, detailed analyses of the energetics of DNA binding and dimerization of OmpR in both its unphosphorylated and phosphorylated state indicate that phosphorylation enhances OmpR dimerization and that this dimerization enhancement is the energetic driving force for phosphorylation-mediated regulation of OmpR-DNA binding. These findings suggest that OmpR phosphorylation mediated activation follows the same paradigm as the other members of the OmpR/PhoB family of RRs in contrast to previously proposed models of OmpR activation.

Studies of transcription regulation are often focused on binding of transcription factors (TFs) to a small number of promoters of interest. It is often assumed that TFs are in great excess to their binding sites (TFBSs) and competition for TFs between DNA sites is seldom considered. With increasing evidence that TFBSs are exceedingly abundant for many TFs and significant variations in TF and TFBS numbers occur during growth, the interplay between a TF and all TFBSs should not be ignored. Here, we use additional decoy DNA sites to quantitatively analyze how the relative abundance of a TF to its TFBSs impacts the steady-state level and onset time of gene expression for the auto-activated Escherichia coli PhoB response regulator. We show that increasing numbers of decoy sites progressively delayed transcription activation and lowered promoter activities. Perturbation of transcription regulation by additional TFBSs did not require extreme numbers of decoys, suggesting that PhoB is approximately at capacity for its DNA sites. Addition of decoys also converted a graded response to a bi-modal response. We developed a binding competition model that captures the major features of experimental observations, providing a quantitative framework to assess how variations in TFs and TFBSs influence transcriptional responses.