ELECTROSTATIC FORCES CONTRIBUTE TO INTERACTIONS BETWEEN TRP REPRESSORDIMERS

The trp repressor of Escherichia coli (TR), although generally conside red to be dimeric, has been shown by fluorescence anisotropy of extrin sically labeled protein to undergo oligomerization in solution at prot ein concentrations in the micromolar range (Fernando, T., and C. A. Ro yer 1992. Biochemistry. 31:3429-3441). Providing evidence that oligome rization is an intrinsic property of TR, the present studies using che mical cross-linking, analytical ultracentrifugation, and molecular sie ve chromatography demonstrate that unmodified TR dimers form higher or der aggregates. Tetramers and higher order species were observed in ch emical cross-linking experiments at concentrations between 1 and 40 mu M. Results from analytical ultracentrifugation and gel filtration chr omatography were consistent with average molecular weight values betwe en tetramer and dimer, although no plateaus in the association were ev ident over the concentration ranges studied, indicating that higher or der species are populated. Analytical ultracentrifugation data in pres ence of corepressor imply that corepressor binding destabilizes the hi gher order aggregates, an observation that is consistent with the earl ier fluorescence work. Through the investigation of the salt and pH de pendence of oligomerization, the present studies have revealed an elec trostatic component to the interactions between TR dimers.