Chromatin superstructure

533 Nature Vol. 262 August 12 1976 news and views Chromatin superstructure from H. G. Davies in understanding the structure of chromatin and eukaryotic chromosomes corutinues apace. Early electron micrographs by Ris and others had shown that the long fibres obtained from chromosomes were either thin, about 10 nm, or thick, about 25 nm, de·pending on the pPesence or absence of chela:ting agents. Hitherto, however, it has not been clear how ,the ,two fibres are related. The past few years have seen the emergence of evidence that chromatin has a subuni.t structure; nuclease digestion of chromatin yields pieces of DNA which are multiples of about 200 base pairs, histones occur as oligomers, and electron micrographs of suitably lysed nuclei show linear arrays of in,te-rconnected roughly spherical particles ~bout IO nm diameter called beads, v-bodies, or nucleosomes. Kornberg has put forward a model for the 10 nm fibre-a flexible chain of repeating structural units, like "beads on a string" (Science, 184, 868; 1974), each bead containing two ·e ach of ,t he four major histones. Finch and Klug (Proc. natn. A cad. Sci. U.S.A ., 73, 1897; 1976) now provide evidence to show thit<t thin fibres about 40 repeat-units long, obtained from rat liver nuclei by brief nuclease digestion, can undergo helical folding to form a solenoidal structure about 30 ,t o 50 nm wide. The evidence for helices is based first on electrnn micrographs which show more-or-less clear striations, separation a,bouit 12-15 nm crossing ithe thick fibre, and second, on new X-ray diffraction data and a reinterpretation of the earlier patterns obtai,ned by Wilkins and collaborators, and Luzzati and Nicolaieff, showing maxima 11.0, 5.5, 3. 7, 2. 7 and 2.2 nm. The key expe,riment is a comparison of the X-ray spectra of thin and thick fibres and the results are different from those obtained previously, due, presumably, to improved methods of preparing chromatin. Sperling and Tardieu (FEBS Lett., 64, 89; 1976) find that the scattering from ,t hin fibres in the region of spacings 40 to 3 nm fits the theoretical curve for a continuous density rod with no peaks at 11 and 5.5 nm . The 11 nm and other reflections appear, however, under conditions where solenoidal structures can be seen in ekctron micPographs. Hence Finch and PROGRESS Klug now propose that ,t he reflections at about 11 nm and higher orders a,rise from the spacings between the turns of solenoids of low pitch angle and not from an interbead spacing of 10-11 nm along the thin fibre as had been universally, and quite reasonably, assumed, and indeed used by Kornberg to support his model. The new explanation of the spectra is similar to that originally proposed by Pardon and Wilkins (J. molec. Biol. , 68, 115; 1972) except that they thought in terms of supercoiHng DNA, -rather than the continuous 10 nm fihre . The data appear to confirm a suspicion held by many that .the repeaiting unit in the thin fibre only becomes visible as vbodies or nucleosomes under special conditions. Those electron microscopists who, for one reason or another, did not see the thin fibres as beaded, but continuous, a result confirmed by Finch and Klug, will be unable to resist a smile at this latest turn of events. Another key observation is that by Carpenter et ·al. (Nucleic Acids Res., 3, 1739; 1976) on neutron diffraction from oriented fibres of chromatin. The meridionally oriented reflection at about 10 nm, is now seen to be split, forming a cross pattern wi,th semimeridional angle of 8° to 9°. This is a sure sign to the cognoscenti that it originates from a helix. They propose as a model a coil of nucleosomes of pitch 10 nm and an outer diameter about 30 nm, dimensions corresponding to about six nucleosomes per turn of ,t he helix. Of course, such a spHtting of this meridional .r eflection could also be expected in the solenoidal model formed from a continuous 10 nm thread. On the basis of electron microscopy of sections, we have described threadlike units wHhin condensed interphase chromosomes, or chromatin bodies. These "superunit threads" about 28.0 nm diameter, are clearly recognisable because they tend to line up into layers at the surfaces of nuclei, and can also form monolayers, width independent of species, plant and animal. Davies and Haynes (J. Cell Sci., 21, 315; 1976) now report that nucleoli are attached ,to the nudear envelope by a monolayer of these superuniits. We have suggested © 1976 Nature Publishing Group that the superunit thread is formed from some type of helical arrangement of a subunit thread-"close-packed beads-on-a-string". This would account for the earlier observation that the unit appears tubular. Finch and Klug comment that their solenoidal model being of similar geometry can be equated with superunit thPeads found within nuclei. Much clearly remains to be learned about the detailed molecula,r structure of these proposed helically arranged repeating units, how they aPe further folded up in mitotic chromosomes and how they unfold in those pal'ts of interphase nuclei whePe RNA transcription occurs. Also the way the DNA molecule is folded around the repeating unit is not dear, but results on sections suggest the repe,ating unit is asymmet,ric with more DNA on an inward pointing face. The spiralling of mitotic chromosome,s and the helical nature of the DNA molecule have long been known and the a bove results will come as no surprise to those who believed that there were orders of coiling in between. The point is that speculation is now being replaced by fact. D 1 A hundred years ago TN the Bulletin International of August 3, M. de Tastes relates some interesting particulars of a waterspout (trombe) which was observed near Tours, on May 25 1876. It first appeared as a mass of whitish vapour against a background of dark-coloured clouds, which gradually assumed the form of an inverted cone pointing to the ground, and terminating in a long sinuous band. A whitish sinuous column soon appeared suddenly between it and the ground, and rapidly enlarged upwards, the whole phenomenon soon assuming the appearance of two cones united at their summits. The lower cone, at first lightish-coloured and in a certain degree transparent, gradually assumed a darker shade, which was propagated from the base towards the summit. from Nature, 14, August 10, 320, 321; 1876.  (...truncated)