Binding of four repressors to double-stranded tet operator region stabilizes it against thermal denaturation.

Abstract

The investigation of protein–DNA interactions benefits from methods for the dissection of chromosomal DNA into handy fragments and their subsequent preparation in large amounts1,2. Of particular interest are proteins active in gene regulation and their interaction with the control sequences of the respective genes3. Recently, we reported4 the purification of the molecular components of the control elements from the tetracycline-resistance (tet) gene located on the transposon Tn10. The Tet repressor inhibits transcription of the tet gene and its own gene4. When the tet operator was prepared on a 187 base pair (bp) DNA fragment4, the Tet repressor was found to bind specifically to this fragment with a stoichiometry of four Tet repressors per DNA fragment4. Tetracycline inhibits this binding4 and operates in vivo as an inducer for the expression of the Tn10-encoded tetracycline resistance5,6. We now report thermal denaturation experiments of the Tet repressor–tet operator complex and demonstrate independently that four Tet repressor molecules bind to the 187 bp DNA and stabilize a 125 bp double-stranded DNA sequence against thermal denaturation.