Monovalent Cation Binding by Curved DNA Molecules Containing Variable Numbers of A-Tracts
Yongjun Lu 1 and Nancy C. Stellwagen 2*
1 The University of Iowa
2 University of Iowa
* To whom correspondence should be addressed. E-mail: nancy-stellwagen{at}uiowa.edu.
Submitted on September 3, 2007
Revised on October 10, 2007
Accepted on 18 October 2007
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Abstract |
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Monovalent cation binding by DNA A-tracts, runs of four or more contiguous adenine or thymine residues, has been determined for two curved ~200 bp restriction fragments, one taken from the M13 origin of replication and the other from the VP1 gene of SV40. These two fragments have previously been shown to contain stable, centrally located bends of 44° and 46°, respectively, located within ~60 basepair "curvature modules" containing 4 or 5 irregularly spaced A-tracts. Transient electric birefringence measurements of these two fragments, sequence variants containing reduced numbers of A-tracts in the SV40 curvature module, or changes in the residues flanking the A-tracts in the M13 curvature module, have been combined with the free solution electrophoretic mobilities of the same fragments, using known equations, to estimate the effective charge of each fragment. The effective charge is reduced, on average, by one-third charge for each A-tract in the curvature module, suggesting that each A-tract binds a monovalent cation approximately one-third of the time. Monovalent cation binding to two or more A-tracts is required to observe significant curvature of the DNA helix axis.
Key Words:
A-tracts, DNA, effective charge, free solution mobility, monovalent cation binding, transient electric birefringence
