Electron spin labeling reveals the highly dynamic N-terminal arms of the SOS mutagenesis protein UmuDElectronic supplementary information (ESI) available: Protein purification protocols, lineshape fitting procedures and best-fit parameters for the spectra shown in Fig. 2 are available. See DOI: 10.1039/c1mb05334e

Electron paramagnetic resonance (EPR) spectroscopy was used to probe the conformational dynamics of the N-terminal arms of the umuD gene products. We determined that the arms of UmuD 2 display a large degree of motion, are largely unbound from the globular C-terminal domain, and that the free energy of dissociation is +2.1 kJ mol −1 . Electron paramagnetic resonance spectroscopy with site-directed spin labeling shows that the N-terminal arms of the SOS response protein UmuD are highly dynamic.