The binding of actinomycin D-3H to heterochromatin as studied by quantitative high resolution radioautography

Studies on corneal endothelial growth and repair. II. increased transcription as detected by incorporation of 3H-uridine and 3H-actinomycin D

Endothelial cells from injured frog corneas undergo increased 3H-uridine and 3H-actinomycin D (3H-AMD) incorporation as judged by autoradiography. The increase in 3H-AMD binding occurs when living endothelium is labeled in vitro or when fixed preparations are exposed to the drug. The changes in 3H-AMD incorporation detected by the two methods are comparable (55 and 62% for living and pre-fixed tissue respectively). However, when fixed endothelium is also de-histonized with 2 N HCl, differential binding of 3H-AMD is eliminated. This result suggests that the enhanced incorporation of 3H-AMD into nuclei is at least partly due to a modification in the association of chromosomal proteins with DNA and not entirely to cell permeability changes that may accompany wound repair. This contrasts with observations of cells that are killed outright by the injury. Such cells bind very large amounts of 3H-AMD compared with their living neighbors. Here the difference in incorporation is eliminated by prefixation. Thus, in the dead cells increased binding may be due to a reduction of cell surface permeability barriers which accompanies cell morbidity.

Labeling and other effects of actinomycin D on human chromosomes

(3)H-actinomycin D, a guanine-binding agent, labels fixed human chromosomes nonrandomly. Actinomycin D added in G2 inhibits secondary constrictions and breaks chromosomes. There is some tendency for label to be concentrated at the ends of chromosomes and near the centromere. Labeling with (3)H-thymidine in the late stage of DNA synthesis shows a different pattern and in general lacks the telomeric concentrations. The sites of actinomycin D-induced breaks do not show good correspondence with the sites of actinomycin D label.