Résumé
LicT belongs to a family of bacterial transcriptional antiterminators, which control the expression of sugar metabolizing operons in response to phosphorylations by the phospho-transferase system. Previous studies of LicT have revealed the structural basis of RNA recognition by the dimeric N-terminal CAT domain on one hand and the conformational changes undergone by the duplicated regulation domain (PRD1 and PRD2) upon activation on the other hand. To investigate on the mechanism of signal transduction between the effector and regulation modules, we have undertaken the characterization of a fragment comprising the CAT and PRD1 domains, and the linker in between. Comparative experiments including RNA binding assays, NMR spectroscopy, limited proteolysis, analytical ultracentrifugation and circular dichroism were conducted on native CAT-PRD1 and on a constitutively active CAT-PRD1 mutant carrying an Asp99Asn substitution in PRD1. We show that in the native state, CAT-PRD1 behaves as a rather unstable, RNA-binding deficient dimer, in which the CAT dimer interface is significantly altered and the linker region is folded as a trypsin-resistant helix. In the activated mutant form, the CAT-PRD1 linker becomes protease-sensitive, the helix content decreases and the CAT module adopts the same dimeric conformation as in isolated CAT, thereby restoring the affinity for RNA. From these results, we propose that a helix-to-coil transition in the linker acts as the structural relay triggered by the regulatory domain for remodeling the effector dimer interface. In essence, the structural mechanism modulating LicT RNA antitermination activity is thus similar to that controlling the DNA-binding activity of dimeric transcriptional regulators.