By contrast, active chromatin does contain an excess of such nuclear polyanions as RNA, phosphoproteins, phospholipids, and non-histone residual proteins. Steroid hormones appear to bind to components of active chromatin during hormone stimulation of nuclear RNA synthesis (10). When added to incubations of isolated repressed chromatin, these nuclear polyanions are each capable of increasing the rate of rate of RNA synthesis within repressed chromatin to the basal level found within isolated active chromatin, but they have no effect on incubations of isolated active chromatin (1). A species of nuclear RNA is particularly effective in such de-repression of chromatin RNA synthesis (1).

These results suggest that the DNA within both active and condensed chromatin is in an electrostatic complex with repressor histones, and that it is nuclear polyanions which effect a de-repression of RNA synthesis by antagonizing the DNA-histone interaction within active chromatin (1). Since RNAs isolated from diverse sources have the same phosphorus content and polyanionic character (11), the greater effectiveness of a species of nuclear RNA in chromatin de-repression suggests that either the specific nucleotide base sequence or the secondary structure of de-repressor RNA plays a part beyond that of the polyanionic character in such de-repression , when compared with other RNAs tested (1). The resistance of active chromatin to further de-repression by added RNA (1) also suggests that specific sites on the DNA genome are already occupied by specific de-repressor RNA.

A species of nuclear RNA has recently been isolated from rat liver which is characterized by a base composition similar to that of DNA, but with sedimentation and metabolic properties which distinguish it from messenger RNA (12). It is known that during RNA synthesis, only one strand of the DNA double helix is transcribed (13-15). The remaining complementary DNA strand has no known function during RNA synthesis (16). Tritium exchange investigations have demonstrated a constant opening and closing of the DNA double helix (17). The magnitude of hyperchromicity observed on heating DNA while it is within active chromatin is less than that observed on heating the same DNA after after its isolation from active chromatin, or on heating DNA while within repressed chromatin (Fig. 1, ref. 1). Such reduced thermal hyperchromicity indicates that a part of the DNA within active chromatin is in the single-stranded state, and that this partially single-stranded DNA reverts to the double stranded state as the histones and polyanions of active chromatin are removed during DNA isolation.

These results suggest that within active chromatin, repressor histones are partially displaced from portions of the DNA genome by nuclear polyanions (Fig. 1):

Fig. 1. Model of specific de-repression of RNA synthesis within active chromatin. Polycationic histone repressors (dark blocks) inhibit the function of DNA as a template for RNA synthesis. Nuclear polyanions partially displace histones from a portion of the DNA genome , allowing specific de-repressor RNA to hybridize with one strand of specific DNA, and freeing the complementary DNA strand for specific messenger RNA synthesis.

Such displacement of histones may allow specific de-repressor RNA to hybridize with a single strand of DNA, freeing the complementary DNA strand for the synthesis of specific messenger RNA.

This work was performed during the tenure of a research career development award (CA-17857) from the
U.S. Public Health Service.

I thank Drs. A.E. Mirsky, V.G. Allfrey, V.R. Potter, H. Swift, and H. Szybalski for their advice.

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