Structure and activation mechanism of Staphylococcus aureus response regulator ArlR by solution NMR.

Two-component systems are the predominant means by which bacteria sense and respond to changes in their environment, utilizing a basic stimulus-response coupling mechanism. In Staphylococcus aureus, ArlRS is a crucial two-component regulatory system involved in biofilm formation and virulence. The response regulator ArlR is composed of a C-terminal DNA-binding effector domain linked to an N-terminal receiver domain that is activated upon phosphorylation by ArlS, the sensor histidine kinase. Crystal structures of the ArlR effector domain reveal that unlike the canonical OmpR/PhoB DNA-binding domain structures, which crystalize as monomers, the ArlR DNA-binding domain undergoes dimerization via N-terminal β1-β2 domain swapping. However, this configuration prevents the effector domain from being properly oriented for DNA binding. Rather a 60- to 90-degree twist of the monomeric units along the dimeric interface would be needed to position the recognition helices correctly to bind a DNA inverted repeat. In our present study, NMR structures demonstrate that the ArlR effector domain folds as a monomer in solution rather than a domain-swapped homodimer. Moreover, domain swapping of the DNA-binding effector domain does not occur upon activation and dimerization of the receiver domains using the phosphomimic beryllium trifluoride. Instead, the effector domains remain separate in the activated dimer but are rigidly positioned by inter-domain interactions present in the monomer, as indicated by 15N NMR relaxation experiments. Still, solvent amide exchange studies point to structural instability of the central β-sheet that allows for the domain-swapping seen upon crystallization, suggesting that a swapped effector domain dimer could still potentially form under different conditions. Cross-talk between domains adds a potential environment-sensitive regulatory control.