Abstract
ChxR is an atypical two-component signal transduction response regulator (RR) of the OmpR/PhoB subfamily encoded by the obligate intracellular bacterial pathogen Chlamydia trachomatis. Despite structural homology within both receiver and effector domains to prototypical subfamily members, ChxR does not require phosphorylation for dimer formation, DNA binding or transcriptional activation. Thus, we hypothesized that ChxR is in a conformation optimal for DNA binding with limited interdomain interactions. To address this hypothesis, the NMR solution structure of the ChxR effector domain was determined and used in combination with the previously reported ChxR receiver domain structure to generate a full-length dimer model based upon SAXS analysis. Small-angle scattering of ChxR supported a dimer with minimal interdomain interactions and effector domains in a conformation that appears to require only subtle reorientation for optimal major/minor groove DNA interactions. SAXS modeling also supported that the effector domains were in a head-to-tail conformation, consistent with ChxR recognizing tandem DNA repeats. The effector domain structure was leveraged to identify key residues that were critical for maintaining protein - nucleic acid interactions. In combination with prior analysis of the essential location of specific nucleotides for ChxR recognition of DNA, a model of the full-length ChxR dimer bound to its cognate cis-acting element was generated.
Highlights
Two-component signal transduction systems (TCS) are a fundamental mechanism employed by bacteria for rapid adaptation to environmental changes
TCS typically consist of a membrane-bound sensor histidine kinase (HK) and an associated response regulator (RR)
Intensity at q = 0 radius of gyration (Rg) value was obtained after extrapolation to infinite dilution; single values were calculated from Guinier fit using a q*Rg,1.6 Rg values were calculated from the P(r) function by the program SCATTER [30]
Summary
Two-component signal transduction systems (TCS) are a fundamental mechanism employed by bacteria for rapid adaptation to environmental changes. TCS typically consist of a membrane-bound sensor histidine kinase (HK) and an associated response regulator (RR). The sensor kinase undergoes an autophosphorylation event from which the phosphoryl group is transferred to the receiver domain of a cognate RR. Phosphorylation of the RR promotes oligomerization, stabilizing the active form of the protein. The majority of response regulators contain a DNA-binding domain that alters gene expression in response to phosphorylation [1]. The functions of RRs involve a diverse array of responses, including drug resistance, motility, osmoregulation, pathogenic host invasion and phosphate uptake, among others [2]. RRs are subdivided into families based upon the structure/function of their DNA binding domains. The largest subfamily of RRs (OmpR/PhoB) is comprised of a winged helix-turn-helix domain [3]
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