Crystal structure of the effector-binding domain of the trehalose-repressor of Escherichia coli, a member of the LacI family, in its complexes with inducer trekalose-6-phosphate and noninducer trehalose

The crystal structure of the Escherichia coli trehalose repressor (TreR) in a complex with its inducer trehalose-6-phosphate was determined by the met hod of multiple isomorphous replacement (MIR) at 2.5 Angstrom resolution, f ollowed by the structure determination of TreR in a complex with its nonind ucer trehalose at 3.1 Angstrom resolution. The model consists of residues 6 1 to 315 comprising the effector binding domain, which forms a dimer as in other members of the LacI family. This domain is composed of two similar su bdomains each consisting of a central beta-sheet sandwiched between alpha-h elices. The effector binding pocket is at the interface of these subdomains . In spite of different physiological functions, the crystal structures of the two complexes of TreR turned out to be virtually identical to each othe r with the conformation being similar to those of the effector binding doma ins of the LacI and PurR in complex with their effector molecules. Accordin g to the crystal structure, the noninducer trehalose binds to a similar sit e as the trehalose portion of trehalose-6-phosphate. The binding affinity f or the former is lower than for the latter. The noninducer trehalose thus b inds competitively to the repressor. Unlike the phosphorylated inducer mole cule, it is incapable of blocking the binding of the repressor headpiece to its operator DNA. The ratio of the concentrations of trehalose-6-phosphate and trehalose thus is used to switch between the two alternative metabolic uses of trehalose as an osmoprotectant and as a carbon source.