Inhibition of transcription in eukaryotic cells by X-irradiation: relation to the loss of topological constraint in closed DNA loops.

X irradiation was found to inhibit in vivo transcription in mammalian, yeast, insect and avian cells in a dose-dependent manner. Measurements of DNA nicking indicated that about one DNA single-strand break per estimated DNA loop (domain) length is sufficient to explain the effect. The inhibitory effect was partially reversed by post-irradiation incubation of cells. During such incubation DNA nicking was considerably repaired. The size of transcripts was not changed by irradiation. The in vitro (run on) activity of RNA polymerase in nuclei isolated from irradiated cells also was not altered. The dose-response curves were different in various cells, correlating with the reported unequal average domain size of supercoiled DNA (and also replicon size) in diverse organisms.