Intrinsically disordered proteins and protein domains (IDPs) are prevalent in all organisms and make up as much as 40-60 % of a typical eukaryotic proteome. IDPs lack stable secondary and tertiary structure and instead exist as heterogeneous ensembles of interconverting conformations. IDPs participate in several biological processes such as signal transduction cascades and transcriptional regulation, though the mechanisms by which they are able to function despite being natively disordered remain poorly understood. Solution NMR spectroscopy has made seminal contributions to our understanding of IDPs because of its ability to characterize structure and dynamics at atomic resolution in highly heterogeneous systems. We show using chemical exchange saturation transfer (CEST) NMR that the intrinsically disordered DNA binding domain of the Escherichia coli cytidine repressor (CytRN) transiently adopts a globally folded conformation (excited state). The structure of this excited state, calculated using multinuclear chemical shifts and residual dipolar couplings, is a three-helix bundle with a helix-turn-helix motif that closely resembles the DNA-bound state of CytRN. We also show using double resonance DANTE-CEST and Carr-Purcell-Meiboom-Gill relaxation dispersion data that DNA molecules select the excited state and not the disordered form for binding. This regulatory disorder-to-order transition seen in CytRN is reminiscent of a lock-and-key model, except that the structurally complementary conformation is not the native state, but is instead accessed via thermal fluctuations. This regulatory switch in CytRN is an excellent model system for testing the effect of mutations on IDP structure and function, and I will also discuss some of our recent data that highlight the non-intuitive effects of point mutations on the folding and binding of CytRN.