Low Mr RNAs of chicken muscle cells and their interaction with messenger and ribosomal RNAs

Cytoskeleton involvement in the distribution of mRNP complexes and small cytoplasmic RNAs

These studies were designed to determine whether small cytoplasmic RNAs and two different mRNAs (actin mRNA and histone H4 mRNA) were uniformly distributed among various subcellular compartments. The cytoplasm of HeLa S3 cells was fractionated into four RNA-containing compartments. The RNAs bound to the cytoskeleton were separated from those in the soluble cytoplasmic phase and each RNA fraction was further separated into those bound and those not bound to polyribosomes. The four cytoplasmic RNA fractions were analysed to determine which RNA species were present in each. The 7 S RNAs were found in all cytoplasmic fractions, as were the 5 S and 5.8 S ribosomal RNAs, while transfer RNA was found largely in the soluble fraction devoid of polysomes. On the other hand a group of prominent small cytoplasmic RNAs (scRNAs of 105-348 nucleotides) was isolated from the fraction devoid of polysomes but bound to the cytoskeleton. Actin mRNA was found only in polyribosomes bound to the cytoskeleton. This mRNA was released into the soluble phase by cytochalasin B treatment, suggesting a dependence upon actin filament integrity for cytoskeletal binding. A significant portion of several scRNAs was also released from the cytoskeleton by cytochalasin B treatment. Analysis of the spatial distribution of histone H4 mRNAs, however, revealed a more widely dispersed message. Although most (60%) of the H4 mRNA was associated with polyribosomes in the soluble phase, a significant amount was also recovered in both of the cytoskeleton bound fractions either associated or free of polyribosome interaction. Treatment with cytochalasin B suggested that only cytoskeleton bound, untranslated H4 mRNA was dependent upon the integrity of actin filaments for cytoskeletal binding.

Isolation and characterization of a translation inhibitor from human term placenta

An inhibitor of protein synthesis has been isolated from free cytoplasmic ribonucleoprotein particles of human term placenta. The inhibitor is resistant to phenol, DNase, proteinase K, and heating at 100 degrees C, but is sensitive to alkaline hydrolysis. These data suggest that the inhibitor is RNA. Experiments provide evidence that this preparation contains no RNase contaminant and does not induce an RNase in this assay system. Three lines of evidence suggest that the inhibitor acts at the initiation of protein synthesis in the wheat germ translation system. First, a lag occurs before cessation of translation when the inhibitor is added to translating polyribosomes. This lag is identical to that seen upon the addition of aurintricarboxylic acid, a known inhibitor of initiation. Second, sucrose gradient analyses demonstrate that, when the inhibitor is present at the start of translation, 40 S complexes form, but neither 80 S complexes nor polyribosomes are seen. Third, gradient analyses show that, when the inhibitor is added to translating polyribosomes, 40 S complexes accumulate with a progressive loss of polyribosomes. Finally, the extent of inhibition depends upon the amount of wheat germ extract added to the reaction mixture and not the amount of mRNA present. This suggests an interaction between the inhibitor and a component of the wheat germ extract.