title: On the heterogeneity of native ribosomal subunits in Ehrlich-ascites-tumor cells cultured in vitro.
abstract: Native small ribosomal subunits in cultured Ehrlich ascites tumor cells, analyzed by high-resolution CsCl isopycnic centrifugation, consist of at least five classes of particles. These particles with buoyant densities of 1.39, 1.42, 1.45, and 1.51 g/cm3 were designated as SI, SII, SIII, SIV and SV, respectively. They were different from the ribosome-derived 40-S subunits which have a density of 1.54 g/cm3. Native large ribosomal subunits consist of at least two classes of particles with densities of 1.57 g/cm3 (LI) and 1.59 g/cm3 (LII), respectively. The ribosome-derived 60-S subunits have the same buoyant density as LII particles. Labeling of Ehrlich ascites cells with radioactive uridine and a subsequent chase in the presence of RNA-synthesis inhibitors shows that radioactivity was incorporated first in precursor particles with a density of 1.48 g/cm3 and then subsequently appeared in SIII and SII particles. Met-tRNAf is found exclusively on native 40-S particles with densities of 1.42 and 1.49 g/cm3. This result has been observed in cells labeled with [35S] methionine in vivo as well as with tRNA charged in vitro. The possibility that SII particles contain 40-S initiation complexes is discussed.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002286:Carcinoma, Ehrlich Tumor / Q000473:pathology*; D002458:Cell Fractionation; D002478:Cells, Cultured; D002499:Centrifugation, Density Gradient; D003593:Cytoplasm / Q000378:metabolism; D008715:Methionine; D012343:RNA, Transfer / Q000378:metabolism; D012270:Ribosomes / Q000378:metabolism / Q000648:ultrastructure*
chemical_list: D012343:RNA, Transfer; D008715:Methionine