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

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• Polydeoxynucleotides with repeating adenosine tracts are common in genomes.
• The ability of these sequences to form nucleosomes, the basic units of DNA packaging, is not fully understood.
• Previous studies suggested that long stretches of adenosine might hinder nucleosome formation.

Purpose of the Study:

• To synthesize and characterize synthetic double-stranded polydeoxynucleotides with varying lengths of contiguous adenosine bases.
• To investigate the nucleosome-forming ability of these synthetic DNA polymers.
• To determine if adenosine-rich sequences have functions independent of nucleosome structure.

Main Methods:

• Synthesis of poly[d(AnT).d(ATn)] with n ranging from 3 to 11.
• Circular dichroism spectroscopy to analyze polymer conformation.
• Nucleosome reconstitution using histone exchange and low-salt procedures.
• Competitive assays to compare reconstitution efficiency.

Main Results:

• Polymers with adenosine tracts up to n=11 successfully formed nucleosomes via histone exchange.
• Even a 150 base-pair poly[d(A.T)] sequence, previously thought refractory, assembled into nucleosomes using low-salt exchange.
• The homopolymer reconstituted into nucleosomes comparably to heterogeneous DNA sequences.

Conclusions:

• Resistance to nucleosome formation does not correlate with the appearance of poly[d(A.T)]-like features in the bulk of nucleosomal DNA.
• Highly adenosine-rich sequences can form nucleosomes, indicating their in vivo function may lie elsewhere.
• The findings challenge the notion that adenosine-rich tracts are inherently incompatible with nucleosome formation.