Conformation of DNA in chromatin reconstituted from poly [d(A-T)] and the core histones

Abstract

Present results provide direct evidence of the nature of a conformational change in DNA when nucleosomes are formed from core histones and poly fd(A-T)]. First, we have found some features which have characteristic aspects of the A like conformation of DNA. Thus, an increased contribution due to a sugar conformation close to C3'-endo puckering is detected in the Raman spectra. In addition, the circular dichroism (C.D.) spectra of reconstituted chromatin with poly PI(A-T)j exhibits an increased intensity at about 262 nm.A second feature acquired by poly rd(A-T)l in nucleosome formation from core histones is related to the presence of a negative band at about 280 nm in the C.D. spectra. The nature of this change is correlated with a DNA conformation characterized by a decreased number of base pairs per turn (28,29). This indicates that these two features of reconstituted nucleosomes reflect the presence of two types of DNA conformations, which overall form is of the B type (22,36).

Conformational transitions of poly(dA-dT).poly(dA-dT) in ethanolic solutions

“….poly(dA-dT) and a non-conservative CD spectrum with a strong band at 260 nm appeared (A6260 = 9.5). A similar CD spectrum of poly(dA-dT).poly(dA-dT) has recently been observed in 70 % trifluoroethanol (21). The typical character of the CD spectrum (22) testifies that poly(dA-dT).poly(dA-dT) assumes the A form.…”

(AT)n is an interspersed repeat in the Xenopus genome.

“…(ii) (AT)n fibres can adopt an altered righthanded DNA conformation, the D polymorph Selsing et al, 1975;Millane et al, 1984). However, even though poly(A) tracts in plasmid DNAs are preferentially excluded from nucleosomes during in vitro reconstitution experiments (Kunkel and Martinson, 1981;Prunell, 1982), (AT)n fibres are readily packaged into nucleosomes in vitro (Simpson and Kunzler, 1979;Prevelige and Fasman, 1983;Brahms et al, 1981). This makes it appear unlikely that the altered structure of (AT)n sequences in linear DNA would preclude their incorporation into a nucleosome as is observed for (A)n sequences.…”