Dynamic Consequences of Specificity within the Cytidine Repressor DNA-Binding Domain

The E. coli cytidine repressor (CytR) is a member of the LacR family of bacterial repressors that regulates nine operons with distinct spacing and orientations of recognition sites. Understanding the structural features of the CytR DNA-binding domain (DBD) when bound to DNA is critical to understanding differential mechanisms of gene regulation. We previously reported the structure of the CytR DBD monomer bound specifically to half-site DNA and found that the DBD exists as a three-helix bundle containing a canonical helix-turn-helix motif, similar to other proteins that interact with DNA [Moody, et al (2011), Biochemistry 50:6622-32]. We also studied the free state of the monomer and found that since NMR spectra show it populates up to four distinct conformations, the free state exists as an intrinsically disordered protein (IDP). Here, we present further analysis of the DBD structure and dynamics in the context of full-site operator or nonspecific DNA. DBDs bound to full-site DNA show one set of NMR signals, consistent with fast exchange between the two binding sites. When bound to full-length DNA, we observed only slight changes in structure compared to the monomer structure and no folding of the hinge helix. Notably, the CytR DBD behaves quite differently when bound to nonspecific DNA compared to LacR. A dearth of NOEs and complete lack of protection from hydrogen exchange are consistent with the protein populating a flexible, molten state when associated with DNA nonspecifically, similar to fuzzy complexes. The CytR DBD structure is significantly more stable when bound specifically to the udp half-site substrate. For CytR, the transition from nonspecific association to specific recognition results in substantial changes in protein mobility that are coupled to structural rearrangements. These effects are more pronounced in the CytR DBD compared to other LacR family members.