"We typically picture DNA as the straight double helix structure, but inside cells, DNA exists in supercoiled loops. Understanding the molecular interactions between the supercoils and the enzymes that participate in DNA functions has been technically challenging, so we typically use linear DNA molecules instead of coiled DNA to study the interactions," said study researchers. "One goal of our laboratory has been to study these interactions using a DNA structure that more closely mimics the actual supercoiled and looped DNA form present in living cells." After years of work, the Zechiedrich lab has created small loops of supercoiled DNA. In the current study, their hypothesis was proven correct. As the DNA supercoils inside the nucleus, it twists and folds in different forms. This is the first step of the mechanism that prompts the enzyme for resolving DNA entanglements. Imagine watching the rodeo. Like roping cattle with a lasso, supercoiled looped DNA captures gyrase in the first step. Gyrase then cuts one double-helix of the DNA lasso and passes the other helix through the break to get free.

 

Researchers confirmed the observation of the path of the DNA wrapped in the loop around gyrase using magnetic tweezers, a biophysical technique that allows to measure the deformation and fluctuations in the length of a single molecule of DNA. Interestingly, the "DNA strand inversion model" for gyrase activity was proposed in 1979 by Drs. Patrick O. Brown and the late Nicholas R. Cozzarelli, also in a Science paper, well before researchers had access to supercoiled minicircles or the 3-D molecular structure of the enzyme. This work opens a myriad of perspectives to study the mechanism of this conserved class of enzymes, which are of great clinical value and supports new ideas on how DNA activities are regulated.