John H. Frenster

Laboratory of Cell Biology, Rockefeller University, New York, New York

E-mail:  frenster@euchromatin.net

Recent experiments reveal that the ratio of polycationic histones to DNA is not significantly different within either the repressed or the active chromatin fractions isolated from interphase calf thymus lymphocytes (Nature, 1965, 206: 680). In addition, the relative proportions of each of the distinct types of histones are similar within the two forms of chromatin. By contrast, active chromatin does contain an excess of such nuclear polyanions as RNA, phosphoprotein, phospholipid, and non-histone residual protein. These nuclear polyanions are each capable of increasing the rate of RNA synthesis when added to isolated repressed chromatin, but they have no further effect on RNA synthesis when added to isolated active chromatin. A species of  nuclear RNA is particularly effective in such derepression of chromatin RNA synthesis.

Although the DNA isolated from either form of chromatin is similar to that from the other in base composition, thermal hyperchromicity, and reactivity to anti-DNA antibodies, the DNA while within the active chromatin complex is less well stabilized by its histones to thermal denaturation or to reactivity to anti-DNA antibodies than is the DNA while within the repressed chromatin complex. In addition, the DNA while within the active chromatin complex displays a reduced magnitude of thermal hyperchromicity which is fully restored to normal after isolation of the DNA. 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 data sugest that during derepression of RNA synthesis, polycationic histones which inhibit strand separation are displaced from portions of the of the DNA genome by nuclear polyanions, permitting partial DNA strand separation (Nature, 1965, 206: 1269), hybridization of one DNA strand with derepressor RNA, and synthesis of messenger RNA on the remaining free DNA strand.

Supported in part by a Research Career Development award (CA-17857) from the USPHS.

1. Frenster JH, "Localized Strand Separations within Deoxyribonucleic Acid during Selective Transcription", Nature 208: 894 (Nov. 27, 1965).

2. Frenster JH, "Correlation of the Binding to DNA Loops or to DNA Helices with the Effect on RNA Synthesis", Nature 208: 1093 (Dec. 11, 1965).

3. Frenster JH, "Single-Cell Analysis of DNase I-Sensitive Sites During Neoplastic and Normal Cell Differentiation within Human Bone Marrow", Ann. N. Y. Acad. Sci., 567: 334 (Aug. 4, 1989).

4. Frenster JH, "Opening the Double Helix to Activate DNA", in "DNA, The Double Helix, Forty Years: Perspective and Prospective", Chicago, October, 1993.