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Slow cruciform transitions in palindromic DNA
Summary
Cruciform extrusion in DNA is a slow process, even when thermodynamically favored. This slow formation rate may explain why cruciform DNA structures are rare in bacterial cells.
Area of Science:
- Molecular Biology
- Biophysics
- Genetics
Background:
- Negative supercoiling favors cruciform extrusion in DNA.
- Cruciform structures form in self-complementary DNA regions.
Purpose of the Study:
- To investigate the kinetics and thermodynamics of cruciform extrusion in DNA.
- To determine the rate of cruciform formation and reabsorption.
- To estimate the free energy required for cruciform generation.
Main Methods:
- Studied cruciform extrusion in plasmid pUC7 DNA with a 48-base-pair palindrome.
- Measured extrusion and reabsorption rates at varying temperatures and supercoiling levels.
- Determined the equilibrium between cruciform and linear DNA structures.
Main Results:
- Cruciform extrusion is a very slow process in moderately supercoiled DNA, with half-times of several hours.
- Rates increase with DNA supercoiling but are only slightly faster for longer palindromes.
- Reabsorption of cruciform arms is also slow.
- Free energy for cruciform generation is estimated at 18 kcal/mol.
Conclusions:
- The slow kinetics of cruciform extrusion limit its prevalence in vivo.
- Even when thermodynamically favored, slow formation prevents cruciform DNA from being common in bacterial cells.