Ribosomes are composed of about equal amounts of rRNA (ribosomal ribonucleic acid), and proteins, with a little proportion of lipids and certain metallic ions such as Mg, Ca and Mn. Proteins and rRNA are the major constituents of ribosomes. Their proportion in both types of ribosomes (70S and 80S) is variable.
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Ribosomal proteins in small subunits are prefixed by an alphabet ‘S’ and in large subunits by ‘L’. Most of ribosomal proteins act as enzymes which catalyze protein synthesis at various stages. Ribosomal RNA (rRNA) in 70S and 80S ribosomes are of three types, i.e., in 70S these are 23, 16 and 5S rRNA and in 80S these are 28S rRNA, 18S rRNA and 5S rRNA.
In 70S ribosome, 23S and 5S rRNA are found in 50S subunits, and 16S rRNA occurs in smaller subunit, the 30S subunits. Similarly, 28S and 5S rRNA occur in larger 60S subunit of 80S ribosome, and 18S rRNA is found in the 40S subunit.
Usually, in 70S ribosomes, RNA is about 65%, and in 80S ribosomes, it is about 45%.
All these components are categorized as below:
Ribosomal RNA generally represents more than 80% of the total RNA present in cells. It is represented by a highly folded filament of RNA which may measure upto 7000 A in extended forms. Protein molecules are usually attached to this r-RNA filament. About 60% of total r-RNA (both 28S and 18S) presents a helical configuration like DNA, but its base composition is not like that of Watson – Crick Model of DNA.
The rRNA contains specific number of methyl groups. At least a portion of RNA contains intramolecular hydrogen bonds. Regions of the molecule in the form of hairpin loops forming two stranded helices. Of course, the configuration of rRNA in solution may not be the same as its configuration, where it is associated with ribosomal protein.
Ribosomal RNA is found in three forms: In eukaryotic cells, the RNAs are larger and 18S is found in small subunit, and 28S and 5S in the larger subunit. Eukaryotic ribosomes also have an extra small RNA, which is called 5.8S RNA and it is transcribed in the nucleolus as a single unit along with 18S and 28S RNA. 5S RNA is synthesized outside the nucleolus.
Whereas in prokaryotes three RNA molecules are 16S RNA in the small subunit, and 23S and 5S in the large subunit. The 30S and 50S ribosomal subunits associate to form 70S ribosomes only when they are involved in protein synthesis. The 70S ribosomes frequently form clusters called polysomes.
The 28S ribosomal RNA lies in 60S subunits of the ribosomes having mol. wt. of 1.3 × 106 daltons and another 18S rRNA is found in 40S subunits having mol. wt. 0.6 × 106 daltons. 5S rRNA usually consists of 12 nucleotides thus a very small molecule.
In eukaryotic cells, ribosomes may be free or attached to the ER. It appears that ribosomes attached to ER are active in protein synthesis, while free ribosomes are not active in protein synthesis.
Protein contents of the ribosomes are very complex and designated as core proteins. About 50 – 55 proteins have been isolated. In prokaryotic ribosomes, about 21 proteins are found in smaller 30S subunit and about 34 proteins in the larger 50S subunit. The proteins are core proteins. All the proteins are different with the exception of one that is present in both subunits.
Eukaryotic ribosomes contain a higher content of protein than prokaryotic ribosomes. For instance, Tsoetal, (1958) determined a protein content of 60% for Pea seedling particles and 55% for those from rabbit reticulocytes (compared to 37% for E. coli). Ribosomal proteins are small (7000 to 32,000 dalton in molecular weight) and are rich in basic amino acids.
Ribosomal proteins can be dissociated from the ribosome and these are called split proteins (SP). The split proteins are SP 50 and SP 30. When split proteins are returned to medium containing inactive ribosome cores and incubated at 37 C, active ribosomes can be reconstituted. SP 50 and SP 30 can also be further fractionated into acidic and basic proteins.
Ribosomal enzymatic proteins:
Most of the ribosomal proteins act as enzymes thereby catalyzing protein synthesis. Initiation proteins F1, F2 and F3 initiate the process of protein synthesis, whereas transfer proteins (G – factor, Ts-factor) help in translocation of ribosomes over mRNA and transfer of t-RNA residue from one site of the ribosome to the other site.
Another enzyme peptidyl transferase helps in the transformation of peptide chains to aminoacyl-tRNA. The other enzymes—termination factors R1 and R2 help in the release and termination of completed polypeptide chain. (See table 2).
As a result of washing of ribosomes with NH4CI and subjected them to column chromatography, Ochoa and co-workers (1960) isolated above factors in protein synthesis. Among them, three initiation factors—lF1, 1F2, and 1F3—are loosely associated with 30S subunit. 1F1 factor is a basic protein having molecular weight of 9200 daltons.
It is involved in the binding of F-met-tRNA. IF2 factor is also a protein of mol. wt. 80000 daltons and contains — SH groups which help in binding with GTP. Third protein factor—1F3, does not require GTP and is involved in binding mRNA to 30S subunits. It is a basic protein having molecular weight of 30,000 daltons. 1F3 may also act as dissociation factor for 70S ribosomes.
Ochoa et al., (1972) have further reported interference factors (i) in bacteria E. coli. These factors bind to IF3 factor, change its specificity and thus regulate the translation of genetic message at the beginning.
Elongation factors are essential for the elongation of polypeptide chain. These are EFG (also called G factor or translocase) and EFT factor. As described earlier EFG or G factor is involved in the translocation of mRNA. In E. coli, it consists of single polypeptide chain having mol. wt. of 72,000 daltons. EFG + GTP promotes the translocation of newly elongated peptidyl-tRNA.
Another EFT factor has two kinds of proteins namely Tu (temperature-unstable) and Ts (temperature stable). The EFTu factor + GTP form complex with aminoacyl-tRNA before its binding to the acceptor site of ribosome being catalyzed by Ts factor.
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Moreover, 50S ribosomal subunit has an enzyme-peptidyl synthetase or peptidyl transferase associated in the formation of peptide bond. Termination factors R> and R2 are releasing proteins helping in liberating the polypeptide chain.