Nucleic Acids Research, 1986, Vol. 14, No. 12 5099-5109
© 1986
Articles |
Role of divalent cations on DNA polymorphism under low ionic strength conditions
Department of Pharmaceutical Chemistry, School of Pharmacy, University of California San Francisco, CA 94143, USA
Received February 21, 1986. Revised May 19, 1986. Accepted May 19, 1986.
We have examined the confonnational properties of poly(dG-m5dC) under a variety of low salt conditions and sample preparations. Extensive dialysis against 0.5 mM Na-cacodylate resulted in a left-handed polynucleotide conformation as determined by circular dichroism, in agreement with recently reported results. Similarly, extensive dialysis against Tris-EGTA also led to a left-handed conformation. Dilution of these samples led to a transition to the right-handed conformation. More stringent treatments such as dialysis followed by passage over an ion exchange column also resulted in a right-handed conformation. When these various solutions were examined using atomic absorption spectroscopy, significant levels of Mg were observed (> 190 per 1000 nucleotldes) in all samples showing a left-handed form, while much lower levels (< 45 per 1000 nucleotides) were found in the low salt samples displaying a right-handed conformation. Addition of MgCl2 to samples in which divalent cations had been almost completely removed led to the reformation of the left-handed form. These results indicate that the left-handed form seen under certain low salt conditions is due to the presence of Mg+2 ions that remain bound to the polynucleotide, even in the presence of EDTA.