Increased binding of operator DNA by trp superrepressor EK49.

The mechanism of superrepression by the mutant trp repressor EK49 was examined. This superrepressor has a glutamic acid-to-lysine change at residue 49. The purified EK49 trp repressor was found to have a 10-fold higher affinity than wild type repressor for trp operator DNA. This increased affinity was shown to be due to a decrease in dissociation rate. The binding of trp operator DNA by EK49 trp repressor in the filter binding assay was more sensitive to high salt concentrations than binding by wild type repressor.