A left-handed (Z) conformation of poly(dA-dC).poly(dG-dT) induced by polyamines

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Abstract
Blocks of potential Z-DNA forming alternating purine-pyrimidine (APP) sequences are widely dispersed in native DNAs. We have studied the effects of naturally occurring polyamines on the conformation of a synthetic APP sequence, poly(dA-dC).poly(dG-dT) by circular dichroism spectroscopy. In the presence of micromolar concentrations of spermidine (125 μM) and spermine (16 μM), this polymer undergoes B to Z transition in low ionic strength (2 mM Na$) buffers. The concentration of polyamines required for B to Z transition increases with Na$ in the buffer and a straight line is obtained on plotting In[Na$] vs. In[spermidine 3$]. However, at concentrations of polyamines higher than those necessary to induce B·to Z transition, Z-DNA converts to ψ -DNA, an ordered, twisted, tight packing arrangement of the double helix. These results suggest a pathway for the transient formation of Z-DNA segments in vivo by interaction of the ubiquitous polyamines with naturally occurring blocks of APP sequences.