Ribosomes of bacteria

Introduction: 

Proteins are the important components of living cells which contribute a lot in cellular activities. Despite the cellular type of organisms, every living cell requires proteins at a defined level to carry on their activities. Multi cellular organisms such as plants and animals cannot even imagine their life without proteins. Acellular organisms such as the viruses depend upon their host cell machinery to synthesize their proteins that are encoded by their genetic material. "Ribosome" is the main site of protein synthesis within a cell. Bacteria also contain ribosomes, which have different types of sub units when compared to those of the Eukaryotes.  Electron microscopy is a boon to the field of Science as it gave us the opportunity to view sub cellular components along with the atomic and sub atomic particles . Amino acids are the building blocks of proteins, which are linked to one another trough peptide bonds to form the complete protein molecule. Ribosomes may be lying either freely within the cytosol or may be attached to the Endoplasmic Reticulum within a multi cellular organism. Since bacteria are very tiny and simple, they do not have to perform multi activities as in the case of plants and animals. This makes a line of differentiation between the cell organelles of uni and multi cellular organisms and becomes the reason for bacteria not having too many organelles like the higher ones. Adding the amino acids to the growing peptide chain until the stop amino acid is encountered, is carried out within the ribosomes and the sequence of the amino acids during protein synthesis is decided by the messenger RNA(mRNA).

Bacterial Ribosomes:

The ribosomes of all the three domains of life are almost similar in structure and differ mainly in the type of ribosomes they possess. Ribosomes are classified into two types namely 70S and the 80S ,on the basis of their sedimentation coefficient denoted by the Svedberg unit "S". Molecular separation techniques such as the centrifugation separate the molecules on the basis of their velocity which is in turn decided by their molecular weight, volume and shape respectively. Two sub units forming a complete ribosome are made up of ribosomal RNA(rRNA) molecules and few proteins(ribosomal proteins). 16S rRNA molecules and 21 ribosomal proteins make up the smaller sub unit of the ribosomes and the larger one has 32S and 5S rRNA molecules and 33 r-proteins respectively. RNA obtained from both the ribosomes and their sub units from the Gram negative bacterium, E-coli, using detergent or phenol methods, gave same result.

The two sub units of the ribosome, along with a transfer RNA molecule, and joining amino acids to the growing peptide chain help in the synthesis of the required proteins. The nucleic acid of the bacteria is first transcribed into the mRNA, which is further translated into the corresponding protein, with all the stages being carried out within the cytosol of the bacteria. Aminoglycosides are a group of antibiotics which make the bacterial ribosomes inactive but do not have any effect on eukaryotic ribosomes. These antibiotics and others structurally similar to them have been found to attack the smaller sub unit of the bacterial ribosome and the presence of adenosine at the position 1408 of their nucleotide sequence, is held responsible for this. Since ribosomes are the only sites for protein synthesis in all types of cells, replacement of the nucleotide, adenosine with the nucleotide guanosine, (at the site of attack by the aminoglycosides) within the eukaryotic ribosomes, protection is rendered to the eukaryotes against these antibiotics.
Earlier, organisms lacking membrane bound cell organelles were addressed as "prokaryotes" and those with proper membranes around their organelles as "eukaryotes". Studies made on the rRNA structures showed that the rRNA sequences were different for not only prokaryotes and eukaryotes, but also for species within the same genus such as the bacteria and the archaea. The 16S rRNA gene of the bacterial ribosomes provided an evolutionary link between different organisms. Along with some proteins, the 16S rRNA forms the smaller sub unit of the bacterial ribosome and the evolutionary difference is due to rapid genetic changes within a small portion of these ribosomal RNA molecules.

Role of ribosomes in protein synthesis:

Protein synthesis, also known as translation is carried out in the presence of tRNA molecules, the amino acids which join the growing peptide chain, Guanosine triphosphate (GTP) and mRNA with a start codon within it, where the translation is initiated and some some of the initiation factors(three factors F1, F2 and F3 also called A, B and C) within the ribosomes. Formation of an initiation complex is the first step of protein synthesis which is carried out by the binding of the fMet-tRNA to the start codon(AUG) of the mRNA. To this complex comes and binds the 50S ribosomal sub unit and forms the 70S initiation complex at the P site of the ribosome. The ribosomes has the P-site(peptidyl) site and the A-site(Aminoacyl) for the binding of the fMet-tRNA and the second coming amino acid containing tRNA's respectively. The initial fMet-tRNA binds to the P-site of the ribosome and the subsequent tRNA's(aminoacyl tRNA) containing the corresponding amino acids encoded by the subsequent codon of the mRNA are brought up and are added to the A-site in the presence of protein factors and GTP's.
Following the above step takes place the addition of the fMet-tRNA and the second aminoayl-tRNA through a peptide bond which is catalyzed by the 50S ribosomal sub unit. After initiation of protein synthesis, elongation of the growing peptide chain occurs by the movement of the fMet-aminoayl-tRNA from the A-site to the P-site occurs, thereby leaving the A-site vacant for being filled up with the next aminoacyl-tRNA. mRNA contains the triplet codons that encode for particular amino acids and the ribosome moves along the mRNA to elongate the growing peptide chain with the formation of the peptide bonds between the amino acids.
As soon as the termination codons are met up by the ribosomes on the mRNA, the process of protein synthesis is ended up and the polypeptide chain is cleaved from the tRNA and the ribosomes and is released for the further steps of conversion into a mature protein. The polypeptide chains undergo many foldings and finally form a complete protein which is active funtionally. UAG, UAA and UGA are the stop codons which when encountered, terminate the translation process and the prime role of ribosomes is to identify the start and the stop codons on the mRNA, besides its other functions. The proteins so formed are transported to their target sites where they carry out their further activity.
The proteins of the bacteria confer them, their pathogenecity and often form the root cause for many deadly diseases. Within the cell wall of the Gram negative bacteria, these proteins are present in the form of lipopolysaccharides or lipooligosaccharides and are named as endotoxins, while the exotoxins(Diphtheriae toxins) are a group of proteins that elicit host responses at a site away from the bacterial growth. These toxins are highly destructive and at a specific dose, they are considered to be fatal to the humans and may not be less effective then the poisons produced by a snake. Bacterial toxins may act as either potent antigens or may even resemble the enzymatic activity in some cases.

References:
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