The effect of DNA architecture on reconstituted nucleosomal arrays is examined for both linear and fully relaxed circular DNA (ring DNA). We performed experimental observation with AFM, and found that ring DNA was more efficient at forming nucleosomes than linear DNA. Based on a consideration of free energy, we suggest that short-range steric interaction between nucleosomes stabilizes the formation of nucleosomes, and that the ring architecture facilitates the formation of nucleosomes by increasing contacts between nucleosomes, through shrinking of the monomolecular DNA chain conformation. (c) 2009 Elsevier B.V. All rights reserved.