Characteristics of tight binding repressors of the lac operon

Abstract

A number of tight binding lac repressor mutants (I tb) have been analyzed. The mutant repressors could be shown to have altered DNA binding properties, which can include an increased affinity for operator DNA, an increased affinity for non-specific DNA, or a decreased affinity for non-specific DNA. Inducer binding of the repressors is normal even though the induction profile of the I tb strains is similar to that of I s mutants. In general, the investigated I tb repressors can be divided into three groups, of which type I shows an increase in affinity for operator as well as for non-specific DNA. A typical example for this type is the well-characterized X86 repressor. Type II repressors show only an increase in affinity for operator DNA. This new type of mutant repressor is of particular interest, since its further analysis might provide information on which amino acid residues of the repressor are directly involved in the specific interaction between repressor and operator. Type III repressors do not show a considerable change in affinity for wild-type operator but might have an increased affinity for O c operators. Some of these repressors also have a decreased affinity for non-specific DNA.