A functional assay in Escherichia coli to detect non-assisted interaction between galactose repressor dimers

Among the Escherichia coli operons repressed from multiple sites on DNA, th e galactose operon is unique: its repression requires an auxiliary protein, HU, to assist cooperative repressor binding to two distant DNA sites. Here we show that GalR can still mediate repression from distant sites in an ar tificial and simplified regulatory region which totally disturbs the organi sation of the natural interactions. This simple and unexpected cooperation of a protein incapable Of self-association in solution might be involved in regulation of the gal operon. Furthermore, the assay may be generalised to detection of rather weak cooperative interactions between DNA-bound protei ns.