Co-operative binding of two Trp repressor dimers to alpha- or beta-centred trp operators.

The alpha-centred trp operator binds one dimer of the Trp repressor, whereas the beta-centred trp operator binds two dimers of the Trp repressor (Carey et al., 1991; Haran et al., 1992). The Trp repressor with a Tyr-Gly-7 substitution binds almost as well as the wild-type Trp repressor to the alpha-centred trp operator, but it does not bind to the beta-centred trp operator. This confirms that Tyr-7 is involved in the interaction between Trp repressor dimers, as seen in the crystal structure (Lawson and Carey, 1993). Further experiments with alpha-centred trp operator variants showed that positions +/-1 of the alpha-centred trp operators play a crucial role in tetramerisation. The two innermost base pairs of the alpha-centred trp operator are not involved in contacts with the dimer of the Trp repressor binding to it. However, substitutions in these positions (T-A to G-T) effectively transform the alpha-centred trp operator into a beta-centred trp operator, and thus encourage the binding of two Trp repressor dimers to this operator. Finally, we demonstrate, with suitable heterodimers, that one subunit of each dimer suffices to bind to a beta-centred trp operator.