Abstract
The synthetic polymer poly(dA)·poly(dT) was long thought to be refractory to nucleosome formation. Several years ago our laboratory demonstrated that the polymr could be mixed with authentic nucleosomes in a low-salt exchange procedure to form a nucleoprotein complex that behaved in a manner identical with that of nucleosomes. Competitive exchange assays at 37°C showed that the homopolymer reconstituted about as well as heterogenous-sequence DNA. However, studies by other laboratories have shown that the conformation of poly(dA)·poly(dT) depends on temperature; the polymer converts from its well-known, atypical structure, found at ambient temperature, to a conformation more closely resembling a canonical Bform as temperature is increased. We have measured the ability of the homopurine·homopyrimidine to form nucleosomes as a function of temperature. It is seen that poly(dA)·poly(dT) forms nucleosomes more strongly as the temperature of the exchange mixture is increased, so that poly(dA)·(dT) outcompetes heterogeneous-sequence DNA for histones at elevated temperatures.
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