Intramolecular signal transmission in a tetrameric repressor of the IclR family.

Members of the IclR family control bacterial genes involved in a number of physiological processes. The IclR-family member TtgV crystallizes as a tetramer, with each TtgV monomer consisting of two domains--a DNA binding domain and an effector recognition domain, which are interconnected by an extended α-helix. When bound to DNA, a kink is introduced so that the extended helix is split in two α-helices (helix-4 and -5). Differential scanning calorimetry studies revealed that TtgV unfolds in a single event, suggesting that the two domains unfold cooperatively. When mutations are introduced in helix-5 that disrupt interactions between Arg98 and Glu102, the thermal unfolding of the TtgV domains becomes uncoupled without compromising effector binding. Two of these mutants (TtgVE102R and TtgVE102A) showed impaired release from target DNA, suggesting that these mutations alter signal transmission. By combining various mutants, we found that the mutations in the connecting α-helix exhibited a dominant effect over mutations in DNA binding and effector binding domains. We propose a model in which the loss of cooperativity of unfolding of TtgV reflects perturbed interdomain communication, and that the transition from the continuous to discontinuous helix may mediate interdomain communication necessary for the proper functioning of TtgV.