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LysR Type Transcriptional Regulators (LTTRs) regulate basic metabolic pathways or virulence gene expression in prokaryotes. Evidence suggests that the activation of LTTRs involves a conformational change from an inactive compact  apo-  configuration that represses transcription to an active, expanded  holo - form that promotes it. However, no LTTR has yet been observed to adopt both configurations. Here, we report the results of structural studies of various forms of the LTTR DntR. Crystal structures of  apo -DntR and of a partially autoinducing mutant H169T-DntR suggest that active and inactive DntR maintain a compact homotetrameric configuration. However, Small Angle X-ray Scattering (SAXS) studies on solutions of  apo -, H169T- and inducer-bound  holo -DntR indicate a different behaviour, suggesting that while  apo -DntR maintains a compact configuration in solution both H169T- and  holo -DntR adopt an expanded conformation. Models of the SAXS-obtained solution conformations of  apo - and  holo -DntR homotetramers in complex with promoter-operator region DNA are consistent with previous observations of a shifting of LTTR DNA binding sites upon activation and a consequent relaxation in the bend of the promoter-operator region DNA. Our results thus provide clear evidence at the molecular level which strongly supports the ‘sliding dimer’ hypothesis concerning LTTR activation mechanisms.

The solution configurations of inactive and activated DntR have implications for the sliding dimer mechanism of LysR transcription factors