Polyamine-mediated conformational perturbations in DNA alter the binding of estrogen receptor to poly(dG-m5dC).poly(dG-m5dC) and a plasmid containing the estrogen response element.

The binding estrogen receptor (ER) to the upstream regions of estrogen-responsive genes, the estrogen response elements (ERE), is of fundamental importance in the regulation of gene expression by estradiol. Multiple cell-specific factors affect ER-ERE binding and modulate the responses of estradiol. We studied the role of polyamines in the recognition of ER, a ligand-activated transcription factor, with a left-handed Z-DNA forming polynucleotide as well as with a plasmid containing ERE. Polyamines are cellular organic cations with multiple functions in cell growth and differentiation. Polyamines induce Z-DNA conformation in alternating purine-pyrimidine sequences. To understand the role of polyamine-induced DNA conformational transition in ER-DNA interaction, we studied the binding of partially purified rabbit uterine ER to poly(dG-m5dC).poly(dG-m5dC). The induction of Z-DNA in the polynucleotide was monitored by circular dichroism and ultraviolet spectroscopic measurements. Binding of ER to poly(dG-m5dC).poly(dG-m5dC) increased from 15% to approx. 50-60% in the presence of 7.5 mM putrescine, 0.5 mM spermidine or 0.25 mM spermine. Maximal binding of ER to the polynucleotide was observed near the midpoint of the B-DNA to Z-DNA transition of the polynucleotide. N1-acetyl spermidine and N1-acetyl spermine facilitated the B-DNA to Z-DNA transition and the binding of ER although they were less effective than the unacetylated analog. Co(NH3)6(3+), a trivalent inorganic cation, also provoked the B-DNA to Z-DNA transition of the polynucleotide and increased its binding to ER. At higher polyamine concentrations, there was an inhibition of ER binding to the polynucleotide. In the presence of polyamines, the binding of ER to a plasmid containing ERE was 2-3-fold higher than that to a control plasmid devoid of ERE. Polyamine-induced facilitation of ER-ERE binding was also confirmed by gel mobility shift assay. Our data indicate that conformational perturbations, similar to that of the early stages of B-DNA to Z-DNA transition, are important in the recognition of ER and ERE.