Form of DNA and the nature of interactions with proteins in chromatin

Abstract

Studies of native chromatins and of isolated nucleosomes (from calf thymus) show that the DNA is in the B form or modified B form. This was determined by Raman spectroscopy of chromatins, of nucleosomes (from calf thymus) and of DNA fibres and directly correlated with X-ray diffraction studies. The Raman spectra of three forms of DNA (A, B and C) have been characteri zed i n fi bres btt by X-ray di ffracti on and Raman spectroscopy on the same sample. In particular, the Raman spectrum of the C form of DNA is characterized by a band of about 870 cma1. For the first time,, chromatins of different origins with increasing content of non-histone proteins have been investigated by Raman spectroscopy. The site of interaction of the non-histone proteins appears to involve the N7 position of guanine while the histone core does not interact at this site. It is proposed that the mechanism of specific recognition in chromatin involves the large groove.

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

“…3). Raman scattering investigations indicate that despite reconstitutioning carried out at relatively low temperature (see Methods formation which supports all our observations that the reconstitution of chromatin occured correctly (22,36). We interpret the decrease in intensity of this adenine band as an indication of the interaction of core histones with the adenine.…”

“…This high resolution spectrum measured at 50 C provides information about the complexity of the backbone conformation, particularly about differences in sugar puckering. It is apparent that in reconstituted 1) The band at 836 cm 1 is always found in DNA fibers and solutions when the puckering of the sugar is in C2'-endo conformation (22,24). A slight displacement of this band to 839 cm'1 is observed and its relative intensity is comparable to that of the band at 820 cm1.…”

“…2) A band at about 808 cm 1 is observed which is characteristic of C3'-endo sugar puckering in DNA fibers in the A form (22,24) and in RNA at about 812 cm 1. This band is only observed in the high resolution spectrum (Fig.…”

“…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). …”

“…This was interfaced to a LSI-11 microprocessor of a TRACOR TN-1710 minicomputer which allows photon counting, operational control of the spectrometer and accumulation of the spectra. Raman-Laser scattering measurements on nucleosomes and on poly £d(A-T)] were performed as described previously (22). The precision of the measurements is + 1 cm1.…”