Gaseous signaling molecules, such as NO, O<sub>2</sub>, CO and H<sub>2</sub>S, are potential mediators of redox stress and are effectors for transcription factor proteins. Heme serves as an effector-binding site in gas-sensing transcription factors, transducing the signal through varying allosteric mechanisms. To understand how small molecule gas binding regulates function, we compare and contrast two CO-sensing proteins from different families, CooA and RcoM, which employ similar coordination changes to promote DNA binding in distinct protein contexts. CooA, a member of the CRP/FNR family, exhibits three states with distinct heme coordination environments and thermodynamic stabilities. Site-directed spin label EPR studies provide evidence that multiple conformations are populated within a single heme coordination state, suggesting that dynamics may play a role in the allosteric mechanism. Spin labels positioned at key points report on local motion, revealing differences among the domains that may be relevant for protein function. RcoM is a unique single component member of the LytTR family, with a heme coordination environment that is similar to and different from that of CooA. Spectroscopic studies conclusively identified the ligands to the heme iron in each of three coordination states. New data reveal that this protein is a dimer, with the heme domain serving as the interface but with no requirement for heme in dimerization. SAXS data and modeling studies provide the first evidence for the structure of the RcoM protein.

Signal transduction mechanisms in heme-dependent gas-sensing transcription factors

253rd National Meeting (2017)

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