A proposed role for 5S ribosomal RNA

5S rRNA and ribosome

5S rRNA is an integral component of the ribosome of all living organisms. It is known that the ribosome without 5S rRNA is functionally inactive. However, the question about the specific role of this RNA in functioning of the translation apparatus is still open. This review presents a brief history of the discovery of 5S rRNA and studies of its origin and localization in the ribosome. The previously expressed hypotheses about the role of this RNA in the functioning of the ribosome are discussed considering the unique location of 5S rRNA in the ribosome and its intermolecular contacts. Based on analysis of the current data on ribosome structure and its functional complexes, the role of 5S rRNA as an intermediary between ribosome functional domains is discussed.

Inhibition of ribosomal subunit association and protein synthesis by oligonucleotides corresponding to defined regions of 18S rRNA and 5S rRNA

Strong complementarity between a conserved sequence near the 3' end of 18S (16S) rRNA of the small ribosomal subunit and a conserved sequence in the 5S rRNA of the large ribosomal subunit supported the suggestion that base-paired interaction between the two RNA molecules could be responsible for the reversible association of ribosomal subunits during protein synthesis. If this were true then oligonucleotides corresponding to defined regions of the 18S and 5S rRNAs should have profound effects on the association of ribosomal subunits and protein synthesis. In this report we show that oligonucleotides, corresponding to a defined region of eukaryotic 18S rRNA, when bound to wheat embryo 60S ribosomal subunits, inhibited association with 40S ribosomal subunits and also inhibited in vitro protein synthesis. Similarly oligonucleotides corresponding to a defined region of 5S rRNA when bound to 40S ribosomal subunits also inhibited the formation of 80S ribosomes and in vitro protein synthesis. The minimum sequences responsible for the inhibition of ribosomal subunit association and in vitro protein synthesis corresponded to the 5' strand of the m2(6)A m2(6)A hairpin structure near the 3' end of 18S rRNA and nucleotides 91-100 of 5S rRNA which are complementary to each other. Sequences at identical positions of Escherichia coli 16S and 5S rRNAs are also complementary to each other.

The role of ribosomal ribonucleic acid in the structure and function of mammalian brain ribosomes

In order to resolve the functional role of intact rRNA in polypeptide chain elongation mouse brain ribosomes were treated with dilute pancreatic or T(1) RNAase (ribonuclease). After RNAase treatment, several physical-chemical properties as well as the functional activity of the ribosomes were measured. RNAase treatment resulted in the extensive hydrolysis of both 18S and 28S rRNA; however, the sedimentation properties of mono-ribosomes were unaltered and more than 90% of the relatively low-molecular-weight RNA fragments remained associated with ribosome particles. Analysis of the ability of RNAase-treated ribosomes to participate in cell-free protein synthesis showed that ribosomes with less than 2% intact rRNA retained more than 85% of their activity in polyphenylalanine incorporation. Proof that the incorporation of phenylalanine by ribosomes with hydrolysed rRNA actually represented active translocation was obtained by the effective inhibition of incorporation by diphtheria toxin. In addition, the oligopeptide products of protein synthesis could be identified by BD (benzoylated diethylaminoethyl)-cellulose column chromatography. Analysis of the size distribution of oligopeptides synthesized by normal and RNAase-treated ribosomes showed no significant differences which indicated that there was no change in the proportion of ribosomes engaged in protein synthesis. Thus strong RNA-protein and protein-protein interactions must serve to maintain the functional integrity of ribosomes even when the rRNA is extensively degraded. The ability of the enzyme-treated ribosomes to efficiently incorporate amino acids clearly demonstrated that ;intact' rRNA is not required for protein-synthetic activity.

Mitochondrial and cytoplasmic ribosomes. Distinguishing characteristics and a requirement for the homologous ribosomal salt-extractable fraction for protein synthesis

1. Mitochondrial and cytoplasmic ribosomes of Euglena gracilis differ in their total RNA and protein content. 2. Mitochondrial ribosomes dissociate to subunits at higher Mg(2+) concentrations than do cytoplasmic ribosomes. 3. A separable 5S RNA is obtained from cytoplasmic and chloroplast ribosomes, but not from mitochondrial ribosomes. 4. For protein-synthesizing activity with a natural mRNA, mitochondrial ribosomes use tRNA from any cell compartment and are partly active with supernatant enzymes from cytoplasm. Cytoplasmic ribosomes are partly active with enzymes and tRNA from mitochondria or chloroplasts. 5. Both mitochondrial and cytoplasmic ribosomes show high specificity for the homologous salt-extractable ribosomal fraction for protein-synthesizing activity.