Structural changes of LicT-PRD. a, structural comparison of native (pink) and mutant (yellow) LicT-PRD monomers, superimposed onto PRD1. The CAT-PRD1 linker and the PRD1-PRD2 hinge region of the native and mutant proteins are colored green and blue, respectively. Structural elements for the PRD2 domain of the mutant LicT monomer are labeled in the corresponding colors. Top inset, zoom on the CAT-PRD1 linker. Bottom inset, zoom on the hinge region connecting PRD1 to PRD2. For clarity, amino acid single-letter codes are used. b, PRD1-PRD2 interface in the native protein. PRD1, PRD2, and the linker connecting both domains are colored light brown, pink, and green, respectively.

The transcriptional antiterminator protein LicT regulates the expression of Bacillus subtilis operons involved in -glucoside metabolism. It consists of an N-terminal RNA-binding domain (co-antiterminator (CAT)) and two phosphorylatable phosphotransferase system regulation domains (PRD1 and PRD2). In the activated state, each PRD forms a dimeric unit with the phosphorylation sites totally buried at the dimer interface. Here we present the 1.95 Å resolution structure of the inactive LicT PRDs as well as the molecular solution structure of the full-length protein deduced from small angle x-ray scattering. Comparison of native (inactive) and mutant (constitutively active) PRD crystal structures shows massive tertiary and quaternary rearrangements of the entire regulatory domain. In the inactive state, a wide swing movement of PRD2 results in dimer opening and brings the phosphorylation sites to the protein surface. This movement is accompanied by additional structural rearrangements of both the PRD1-PRD1 ’ interface and the CAT-PRD1 linker. Small angle x-ray scattering experiments indicate that the amplitude of the PRD2 swing might even be wider in solution than in the crystals. Our results suggest that PRD2 is highly mobile in the native protein, whereas it is locked upon activation by phosphorylation.