CRP FIXES THE ROTATIONAL ORIENTATION OF COVALENTLY CLOSED DNA-MOLECULES

Five minicircles of 284 bp were constructed with a reporter sequence l ocated approximately opposite a CRP binding site. The spacing between the center of the CRP site and this sequence is varied within 1.2 heli cal turns. The reactivity of the reference sequence to DNAse I was det ermined on the minicircles and on the corresponding fragments, in both the absence and presence of CRP. A rigorous mathematical analysis of the data shows that in the absence of CRP no preferred rotational orie ntation of the DNA is observed. In contrast, binding of CRP fixes, in a phase-dependent manner, the rotational orientation of the reporter s equence in the minicircles. This result illustrates the transmission a t a distance along the DNA molecule of a structural modification. Such effects modulate the extent of synergy between activators and polymer ases during the initiation of transcription.