Characterization of nuclear RNA synthesis in Amoeba proteus

The synthesis of RNA in the ameba nucleus has been studied by labeling with 3H-uridine and fractionation in sucrose density gradients. After a relatively short labeling period radioactivity is primarily restricted to the 30S, 19S, (both ribosomal) and 4S regions, with no significant amount of radioactivity detectable in regions above 30S, Various tests appear to exclude the possibility of differential breakdown or loss of RNA larger than 30S during preparative procedures. We conclude that the ameba nucleus does not synthesize the high molecular weight, rapid turnover, intranuclear RNA (HnRNA) found in a variety of metazoans and possibly does not synthesize pre-ribosomal RNA. In a search of the literature we could find no convincing evidence for the synthesis of HnRNA in unicellular eukaryotes. It appears that the line of separation for the occurrence of HnRNA may not be between prokaryotes and eukaryotes but rather between unicellular eukaryotes and multicellular eukaryotes. Experiments employing long term labeling, short term labeling followed by a long “chase”, and starvation of labeled cells demonstrate relatively long-lived RNA in the ameba nucleus. This RNA is found primarily in the 30S and 4S regions of a sucrose density gradient, although significant amounts are present in the region between these size classes. In these studies the prominent 19S peak of labeled nuclear RNA observed with short term labeling is virtually absent, indicating as in other cells a much more rapid exit of 19S RNA from the nucleus in comparison to RNA in the 30S region.