A dynamic duo: Copper metalloregulators and continuous-wave electron spin resonance spectroscopy.

This review highlights the use of continuous-wave electron spin resonance (ESR) spectroscopy to investigate protein site-specific dynamics and metal coordination in bacterial copper metalloregulators. ESR provides direct insight into how metal binding influences conformational flexibility, revealing how similar proteins can employ distinct mechanisms of transcriptional regulation. Studies of the activator CueR and the repressor CsoR show that these systems use fundamentally different strategies to control gene expression. CueR functions as a stable dimer that activates transcription through metal-induced conformational changes rather than oligomeric transitions. In E. coli, Cu(I) binding rapidly increases dynamics in the DNA-binding domain, while in Pseudomonas aeruginosa, CueR exhibits a more gradual, DNA-dependent response that reflects adaptation to alternative copper resistance pathways. In contrast, CsoR exists in a dynamic equilibrium between dimeric and tetrameric states, with oligomerization playing a central role in its repressor mechanism. Small shifts in subunit exchange may fine-tune DNA binding and facilitate dissociation upon Cu(I) coordination. This review demonstrates how ESR spectroscopy can capture the subtle structural and dynamical differences that define metalloregulator function and highlight the diversity of bacterial strategies for maintaining metal homeostasis.