[Contraction of the disordered loop located within C-terminal domain of the transcriptional regulator HlyIIR causes its structural rearrangement].

HlyIIR is the negative transcriptional regulator of the hemolysin II gene from Bacillus cereus. Previous X-ray studies showed that HlyIIR contains a disordered loop (a.a. 170-185) located within the C-terminal domain near dimerization interface. To understand the influence of this region on HlyIIR properties and for a potential improvement in the crystallogenesis of the HlyIIR, we constructed a mutant of HlyIIR in which this disordered region is substituted by a single alanine residue. Biochemical analysis of the mutant indicated that it still forms a dimer but the DNA-binding activity is lost. HlyIIR mutant displayed improved crystallization properties and its structure was determined by X-ray crystallography at 2.1 A resolution. Unexpectedly, the structure shows that the HlyIIR mutant forms an alternative dimer with subunits rotated by 160 degrees. Moreover, there are also changes in the conformation of individual subunits. These dramatic structural rearrangements are caused by changes in the conformation of the segment Pro161-Ser169. We conclude that correct conformation of this segment is principal for maintaining the structure and activity of HlyIIR.