The mechanism of formation of inhibitor-induced ribosome helices in Entamoeba Invadens

Abstract

Helices and aggregates of helices (chromatoid bodies) composed of ribosome-like particles appear in cysts and slow-growing trophozoites of Entamoeba invadens. We found that similar helix aggregates were formed abundantly in actively growing E. invadens trophozoites treated with a variety of direct or indirect inhibitors of protein synthesis. The inhibitor-induced helices appeared cytochemically and ultrastructurally identical to those seen in cysts. Numerous single helices and small arrays occurred randomly distributed throughout the trophozoite cytoplasm within 15 min after treatment with NaF, which rapidly and completely stopped all nucleic acid and protein synthesis. Cycloheximide (CH), which inhibited protein synthesis as effectively as NaF, stimulated aggregate formation more slowly, and only after a delay of 30-60 min. CH temporarily blocked NaF-stimulated aggregate formation. Aggregation was slowest with actinomycin-D, which strongly inhibited RNA synthesis but depressed protein synthesis only slowly. These results suggested that release of ribosomes from mRNA was required for aggregation. Inhibition by CH was reversible, and aggregates disappeared from CH-treated amebas shortly after they were transferred to inhibitor-free growth medium. There was no evidence that helices assembled about a structural organizer within the cell or that the process involved metabolic activity. It was concluded that the inhibitor-induced helices were composed of mature, normally functional ribosomes and that helix formation was a spontaneous and reversible consequence of the accumulation within the cell of free monosomes (or subunits) which were prevented from binding to mRNA. © 1975, Rockefeller University Press., All rights reserved.