Two-component signal transduction systems are modular phosphorelay regulatory pathways common in prokaryotes. In the co-crystal structure of the Escherichia coli NarL signal output domain bound to DNA, we observe how the NarL family of two-component response regulators can bind DNA. DNA recognition is accompanied by the formation of a new dimerization interface, which could occur only in the full-length protein via a large intramolecular domain rearrangement. The DNA is recognized by the concerted effects of solvation, van der Waals forces and inherent DNA deformability, rather than determined primarily by major groove hydrogen bonding. These subtle forces permit a small DNA-binding domain to perturb the DNA helix, leading to major DNA curvature and a transition from B- to A-form DNA at the binding site, where valine on the recognition helix interacts unexpectedly with the polar major groove floor.

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Examination of the binding of FeBABE-conjugated BvgA to the fha promoter of Bordetella pertussis has revealed that three dimers, formed by head-to-head association of monomers, bind one face of the DNA helix from the inverted-heptad primary binding site to the -35 region. The orientation of BvgA monomers within the dimers is the same as that recently demonstrated by X-ray crystallographic methods for a dimer of the C-terminal domain of NarL bound to DNA. Use of FeBABE conjugates of RNAP alpha subunit C-terminal domain showed that binding of this domain is linearly coincident with binding of the BvgA dimers, but to a different helical face. These results reveal a previously undescribed mode of interaction between RNAP alpha-CTD and a transcriptional activator.

NarL is a model response regulator for bacterial two-component signal transduction. The NarL C-terminal domain DNA binding domain alone (NarL(C)) contains all essential DNA binding determinants of the full-length NarL transcription factor. In the full-length NarL protein, the N-terminal regulatory domain must be phosphorylated to release the DNA binding determinants; however, the first NarL(C)-DNA cocrystal structure showed that dimerization of NarL(C) on DNA occurs in a manner independent of the regulatory domain [Maris, A. E., et al. (2002) Nat. Struct. Biol. 9, 771-778]. Dimerization via the NarL(C) C-terminal helix conferred high-affinity recognition of the tail-to-tail promoter site arrangement. Here, two new cocrystal structures are presented of NarL(C) complexed with additional 20mer oligonucleotides representative of other high-affinity tail-to-tail NarL binding sites found in upstream promoter regions. DNA structural recognition properties are described, such as backbone flexibility and groove width, that facilitate NarL(C) dimerization and high-affinity recognition. Lys 188 on the recognition helix accommodates DNA sequence variation between the three different cocomplexes by providing flexible specificity, recognizing the DNA major groove floor directly and/or via bridging waters. The highly conserved Val 189, which enforced significant DNA base distortion in the first cocrystal structure, enforces similar distortions in the two new cocrystal structures. Recognition also is conserved for Lys 192, which hydrogen bonds to guanines at regions of high DNA helical writhe. DNA affinity measurements for model NarL binding sites, including those that did not cocrystallize, suggest a framework for explaining the diversity of heptamer site arrangement and orientation.

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