Leucine-regulated self-association of leucine-responsive regulatory protein (Lrp) from Escherichia coli

Lrp is a global regulatory protein in Escherichia coli that activates expre ssion of more than a dozen operons and represses expression of another doze n. For some operons, exogenous leucine reduces the extent of Lrp action, fo r others it potentiates the effect of Lrp, and for yet other operons it has no effect. In an effort to understand how leucine affects Lrp-mediated exp ression, we examined Lrp self-association and the effect of leucine on self -association using light scattering, chemical cross-linking, and analytical ultracentrifugation. The following results were obtained. (i) Lrp self-ass ociates to a hexadecamer and octamer with the predominant species being hex adecamer at muM concentrations. (ii) Lrp undergoes a leucine-induced dissoc iation of hexadecamer to octamer. (iii) A mutant Lrp lacking 11 amino acid residues at the C terminus does not form higher-order oligomers, suggesting that the C terminus is involved in subunit association. (iv) At nM concent rations, Lrp dissociates to a dimer. It is proposed that leucine regulates the equilibrium between Lrp oligomers and thus Lrp occupancy of sites withi n different operons, leading to diverse regulatory patterns. (C) 2001 Acade mic Press.