Structure of transcriptionally active chromatin: Radiological evidence for requirement of torsionally constrained DNA

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Abstract
Synthesis of α‐ and β‐globin RNA in DMSO‐induced Friend's erythroleukemia cells and synthesis of immunoglobulin γ‐ and k‐chain RNA, total RNA, 5S RNA, and tRNA in mouse myeloma cells (MPC‐11) was inhibited by γ‐irradiation. For all RNA species, synthesis decreased nearly exponentially as a function of radiation dose, whereas RNA size distributions, turnover rates, and specific activities of radioactively labeled RNA were affected only insignificantly. D37 values for the loss of synthesis of various RNA species correspond to target sizes ranging from 21,000 to 53,000 kd, or 30–80 kbp of DNA. These target sizes are several‐fold larger than the structural genes in question; however, they correspond well with the size of DNA loops, or “domains” constrained by the nuclear matrix. The data suggest that the eukaryotic transcription unit is the torsionally constrained chromatin loop, transcription of which may be inactivated, or significantly reduced by a DNA single‐strand break.