was modulated at 400 cps for purposes of phase sensitive detection. The y-axis polarized transition was observed at a frequency of 13.4 + 0.2Mc. This agrees to within the experimental with the frequency of '/2(A-B) = 16 i 3 Mc that is predicted for the y-axis polarized transition, A/,<-+ 02. The second equally intense absorption at 15.5 4 0.2 Mc was found to be z-axis polarized. This agrees satisfactorily with the frequency l/2(B-C) = 15 3 Mc that is predicted for the zaxis polarized transition, 02 +, 413. In these experiments the zero-field signal/ noise ratio was ca. 50/1. The estimated line-width (on a frequency scale) at zerofield is of the order of one megacycle. Further work on this subject is in progress. We are greatly indebted to Dr. C. Heller for helpful discussions.
We have investigated the internal motions of DNA in a nucleosome core particle and chromatin by measuring the nanosecond fluorescence depolarization of intercalated ethidium. Assuming that the observed anisotrophy decay originates from the torsional motion of DNA, we have analyzed the dynamics of DNA in a nucleosome core particle and in chromatin in detail. The results suggest that DNA in a nucleosome core particle has a torsional rigidity similar to that of DNA in solution and that even at the point of the ionic bonds between DNA and a histone octamer the torsional motion of DNA is not completely inhibited. On the other hand, the dynamics of linker DNA in chromatin were found to reflect the overall structural state of the chromatin: the motion of linker DNA was suppressed as the structure of chromatin turned from an extended state to a condensed one. This indicates that, in solenoidal chromatin, nucleosome movements in chromatin are largely suppressed. Furthermore, the result may suggest that the torsional rigidity of linker DNA is increased as it is forced to bend in solenoidal chromatin.
Novel capsomeric complexes, pentamers and hexamers were detected as chemical entities in phage Q beta. Both were composed of identical protein subunits and stabilized by intermolecular disulphide bonds. Their numbers per particle were about 12 for pentamers and about 20 for hexamers--consistent with theoretical expectation from the quasi-equivalent packing of 180 identical subunits in a coat protein shell.
We have studied the dynamics of DNA in nucleosome core particles and in the linker region of chromatin using nanosecond fluorescence anisotropy decay measurements of intercalated ethidium. DNA in the core undergoes torsional motions to the same extent as the linker DNA in extended chromatin. We therefore concluded that the binding of DNA to the histone octamer is relatively weak or limited to a few points; stretches of at least several tens of base pairs exist which can move as freely as DNA in solution.
The structure of chromatin in the presence of Mg2+ ions was examined by circular dichroism and equilibrium dialysis. Circular dichroism (CD) shows that above 260 nm the intensity of the spectrum of DNA in nucleoproteins decreases as the Mg2+ concentration increases. This change is an intrinsic characteristic of DNA since it is also observed in protein-free DNA and has been attributed to a change in the winding angle of base pairs around the DNA axis. Some structural elements of the DNA in the nucleosome core, therefore, are as movable as those of protein-free DNA. The basic organization of H1-depleted chromatin, 146 base pairs (bp) of DNA wound around core histones and a residual 49 bp in the linker region in the repeating unit, is maintained both in the presence and in the absence of Mg2+ ions, as shown by the fact that the CD spectrum of H1-depleted chromatin has the same type of linear combination between the spectrum of protein-free DNA and that of the nucleosome core in 0.2 mM MgCl2-10 mM triethanolamine (pH 7.8) as it has in 1 mM ethylenediaminetetraacetic acid-10 mM tris(hydroxymethyl) aminomethane (pH 7.8). The ellipticity of chromatin shows a smaller decrease relative to the other nucleoproteins and protein-free DNA upon the addition of Mg2+ ions. Therefore, some structural elements of chromatin are apparently somewhat protected against the conformational change induced by these ions. The spectrum of chromatin becomes almost indistinguishable from that of H1-depleted chromatin in 0.2 mM MgCl2.(ABSTRACT TRUNCATED AT 250 WORDS)
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