Electron microscopic studies on HeLa cells exposed to the antibiotic toyocamycin.

The antibiotic toyocamycin (TCM) selectively inhibits the synthesis of ribosomal RNA in different mammalian cells by suppressing the cleavage of the 45 S RNA precursor to 18-28 S RNA (27). Present ultramorphologic studies on HeLa cells exposed to TCM reveal a highly specific response to the drug. When employed in low concentrations, TCM selectively induces an enlargement of the pars fibrosa in the nucleolus of the treated cells. High doses of TCM, however, provoke morphologic changes similar to those observed with the action of antibiotics interfering with the template activity of DNA.