Sequence-directed bent DNA helix is the specific binding site for Crithidia fasciculata nicking enzyme

The sequence-directed bent structure of kinetoplast DNA minicircles specifies a unique binding site for Crithidia fasciculata nicking enzyme. Binding of the purified enzyme to the bent structure results in the formation of a tight enzyme-DNA complex that is highly specific to curved DNA. Recognition of the binding site is not determined by the nucleotide sequence at the site of binding per se but through the specific local variation in the DNA helix geometry. Both dynamic curved structures, which are generated by supercoiling, and static ones, which are sequenced-directed, could support and efficient enzyme-DNA complex formation. Binding interactions are dependent upon the degree of the helix curvature and decrease with the straightening of the binding site. DNase I protection experiments identify distinct domains of enzyme binding within the bent structure and suggest the induction of structural changes within these regions as a result of protein-DNA interactions.