On the structure of cellular and viral chromatin

Abstract

Some of the recent experimental data obtained in our laboratory are briefly reviewed. 1. A mild staphylococcal nuclease digestion of either chromatin or nuclei from mouse Ehrlich tumour cells results in chromatin subunits (mononucleosomes) of three discrete kinds. The smallest mononucleosome (MN₁ contains all histones except H1 and a DNA fragment 140 base pairs long. The intermediate mononucleosome (MN₂) contains all five histones and a DNA fragment 170 base pairs long. The third mononucleosome (MN₃) also contains all five histones, but its associated DNA is longer and somewhat heterogeneous in size (180-200 base pairs). Most of the MN₃ particles are rapidly converted to mononucleosomes MN₂ and MN₁ by nuclease digestion. However, there exists a relatively nuclease-resistant subpopulation of the MN₃ mononucleosomes. These 200 base-pair MN₃ particles contain not only histones but also non-histone proteins and are significantly more resistant to nuclease then even the smaller mononucleosomes MN₁ and MN₂. 2. Nuclease digestion of hen erythrocyte nuclei or chromatin, in which histone H1 is partially replaced by histone H5 produces the mononucleosomes MN₁ and two electrophoretically resolvable kinds of MN₂ mononucleosomes, one containing histone H1 and the other one histone H5. A relatively nuclease-resistant subset of the mononucleosomes MN₃ is preferentially accumulated at later stages of the digestion. 3. Although pancreatic DNase (DNase I) and spleen acid DNase (DNase II) attack the DNA in chromatin in a manner different from that of staphylococcal nuclease, the deoxyribonucleoprotein (DNP) products of digestion are similar for all three enzymes under identical solvent conditions, as revealed by gel electrophoresis of the DNP at low ionic strength. 4. There are eight major kinds of staphylococcal nuclease-produced soluble subnucleosomes (i.e. particles smaller than the mononucleosomes). In particular, the subnucleosome SN₁ is a set of naked double-stranded DNA fragments ca. 20 base pairs long. Subnucleosome SN₂ is a complex of a specific highly basic non-histone protein and a DNA fragment ca. 27 base pairs long. Subnucleosomes SN₇ and SN₈ each contain all of the histones except H1 and DNA fragments ca. 100 and 120 base pairs long, respectively. 5. Nuclease digestion of isolated mono- and dinucleosomes does not produce all of the subnucleosomes. These and related findings indicate that the cleavages required to generate these subnucleosomes result from some aspect of chromatin structure which is lost upon digestion to mono- or dinucleosomes. Nuclease digestion of isolated minichromosomes of Simian virus 40 (SV40) (which contain all five histones including H1) produces mononucleosomes MN₁ and MN₂ but does not produce some of the subnucleosomes or the relatively nuclease-resistant subset of the MN₃ mononucleosomes. 6. The rate of sedimentation of the SV40 minichromosomes (ca. 60S) under 'physiological' ionic conditions (μ ≈ 0.15) is about two times higher than that in a low ionic strength buffer (μ ≈ 0.005). Occurrence of the compact state of the minichromosome critically depends upon the presence of histone H1 and can be irreversibly fixed by treatment with formaldehyde.

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