Poly[d(A · T)]and other synthetic polydeoxynucleotides containing oligoadenosine tracts form nucleosomes easily

Abstract

Synthetic double-stranded polydeoxynucleotides of the general form poly[d(A n T) · d(AT n )], with n ranging from 3 to 11, have been synthesized. The conformation of the polymers was investigated by circular dichroism spectroscopy and the polymers were examined for their ability to form nucleosomes. Although spectra show that a circular dichroism band characteristic of poly[d(A · T)] appears in the polymer family for n ≥ 7, we demonstrate that even polynucleotides with the longest tracts of contiguous adenosine bases ( n = 11) are able to form nucleosomes when reconstituted using a histone exchange procedure. Thus resistance to nucleosome formation does not coincide with the appearance of features similar to that of poly[d(A · T)] over the bulk of the nucleosomal DNA. Furthermore, we show that an approximately 150 base-pair poly[d(A · T)] itself, long thought to be refractory to nucleosome formation, can assemble into such a protein-DNA complex when reconstituted by a low-salt exchange procedure. Competitive assays show that the homopolymer reconstitutes about as well as heterogeneous sequences DNA. Our work, therefore, suggests that highly adenosine-rich sequences in vivo apparently have a function that operates at a level other than that of nucleosome structure.