Crystal structure of an activated form of the PTS regulation domain from the LicT transcriptional antiterminator

The transcriptional antiterminator protein LicT regulates the expression of Bacillus subtilis operons involved in beta -glucoside metabolism. It belon gs to a newly characterized family of bacterial regulators whose activity i s controlled by the phosphoenolpyruvate:sugar phosphotransferase system (PT S). UcT contains an N-terminal RNA-binding domain (56 residues), and a PTS regulation domain (PRD, 221 residues) that is phosphorylated on conserved h istidines in response to substrate availability. Replacement of both His207 and His269 with a negatively charged residue (aspartic acid) led to a high ly active LicT variant that no longer responds to either induction or catab olite repression signals from the PTS. In contrast to wild type, the activa ted mutant form of the LicT regulatory domain crystallized easily and provi ded the first structure of a PRD, determined at 1.55 Angstrom resolution. T he structure is a homodimer, each monomer containing two analogous a-helica l domains. The phosphorylation sites are totally buried at the dimer interf ace and hence inaccessible to phosphorylating partners. The structure sugge sts important tertiary and quaternary rearrangements upon LicT activation, which could be communicated from the protein C-terminal end up to the RNA-b inding domain.