Bacterial Translation

Bacterial Translation

Bacterial translation may be defined as - ‘the specific process via which the critical nitrogenous-base sequence of mRNA affords determination of the amino acid sequence of protein’.

One may precisely observe that in an organism that is particularly devoid of a membrane-enclosed nucleus, both ‘transcription’ and ‘translation’ invariably occur in the cytoplasm. Thus, in an eukaryotic organisms, the process of ‘translation’ actually comes into being in a situation when mRNA gains its entry into the cytoplasm.

Figure : 6.7 illustrates the eight major sequential stages that are intimately involved in the proc-ess of translation, namely :

Stage-1 : Various components that are essentially required to commence the ‘phenomenon of translation’ first come together.

Stage-2 : On the assembled ribosome a transfer RNA (tRNA) carrying the ‘first amino acid’ is duly paired with the start codon on the mRNA ; and a ‘second amino acid’ being carried by tRNA approaches steadily.

State-3 : Critical place on the chromsome at which the very first tRNA sites is known as the P site. Thus, in the corresponding A site next to it, the second codon of the mRNA pairs with a tRNA carrying the second amino acid.

Stage-4 : First amino acid gets hooked on to the second amino acid by a peptide linkage , and the first tRNA gets released.

Note : Nucleotide bases are duly labeled only for the first two codons.

Stage-5 : Ribosome gradually moves along the mRNA until the second tRNA is in the P site, and thus the process continues.

Stage-6 : Ribosome very much continues to move along the mRNA, and thus, newer amino acids are progressively added on to the ‘polypeptide chain’ strategically.

Stage-7 : Ribosome when ultimately gets upto the ‘stop codon’, the duly formed polypeptide is released.

Stage-8 : Last tRNA gets released finally, and thus the ribosome falls apart. Finally, the re-leased polypeptide gives rise to an altogether new protein.

Process of Bacterial Translation : The various steps encountered in the elaborated process of ‘bacterial translation’ are :

(1) Proteins are usually synthesized strategically in the 5′ → 3′ direction, as present in DNA and RNA (i.e., nucleic acids).

(2) First and foremost, the 5′ end of the specific mRNA molecule becomes associated with a ribosome, which being the major cellular machinery that predominantly helps to catalyze the ‘protein synthesis’.

(3) Ribosomal RNA [rRNA] : Ribosomes usually comprise of two subunits ; of which, one being a special type of RNA termed as ribosomal RNA (rRNA) and the other proteins. At the very outset of the process of bacterial translation, the two ribosomal subunits happen to get closer vis-a-vis the mRNA plus many other components engaged in this phenomenon.

(4) Even before the suitable amino acids may be joined together to yield a ‘protein’, they should be adequately ‘activated’ by strategic attachment to transfer RNA (tRNA).

Figure : 6.8(a) represents the various diagramatic sketch of structures and articulated function of transfer RNA (tRNA).

Figure : 6.8(b) depicts the manner whereby each different amino acid having a particular tRNA gets duly attached to its specific tRNA in the course of ‘amino acid activation’ process. However, this attachment may be adequately achieved by the aid of an amino acid activating enzyme together with sufficient energy derived from adenosine triphosphate (ATP).

Figure : 6.8(c) illustrates clearly the way mRNA actually establishes the precise order wherein amino acids are duly linked together to give rise to the formation of a protein. Thus, each and every set of three nucleotides of mRNA, usually termed as codon, evidently specifies (i.e., codes for) a ‘single amino acid’.

Example : The following sequence :

AUGCCAGGCAAA

essentially contains four codons (i.e., four sets of 3 nucleotides of mRNA) codefying for the amino acids viz., methionine (AUG), proline (CCA), glycine (GGC), and lysine (AAA).

In case, the bases are grouped in an altogether different manner, the ‘same sequence’ might specify other amino acids.

Example : AUGC CAG GCA AA

The above sequence duly encodes : cysteine (UGC), glutamine (CAG), and alanine (GCA).

Likewise, AU GCC AGG CAAA

Would rightly encode alanine (GCC), arginine (AGG), and glutamine (CAA).

Reading Frames : In fact, all the above cited ‘groupings’ are known as reading frames. Importantly, a particular reading frame is invariably determined by the inherent strategic position (status) of the ‘very first codon’ of the gene.

(5) The transfer RNA [tRNA] molecules actually help to ‘read’ the so called coded message located strategically on the mRNA.

Anticodon : Anticodon refers to ‘a set of three nucleotides, which is critically positioned on one particular segment of each tRNA molecule, that happens to be complementary to the codon specifically for the ‘amino acid’ being carried by the tRNA [see Fig. 6.8(c)].

(6) It has been duly observed that in the course of ‘translation’, the highly specific ‘anticodon’ of a molecule of tRNA gets intimately H-bonded to the complementary codon strategi-cally located on mRNA.

Example : One may critically observe that a tRNA having the desired anticodon CGA pairs specifically with the mRNA codon GCU. Therefore, the eventual pairing of anticodon and codon may usually take place solely at two sites as indicated by the ribosome, such as :

(a) The ‘A’ or ‘aminoacyl-site’, and

(b) The ‘P’ or ‘peptidyl-site’.

(a) The structure of tRNA is designated in 2D-form. Each ‘box’ represents a ‘nucleotide’. The critical zones of H-bonding between ‘base pairs’ and ‘loops of unpaired bases’ i.e., a typical arrangement to be seen exclusively in RNA molecules.

(b) Activation of ‘each amino acid’ by due attachment to tRNA.

(c) ‘Anticodon’ by tRNA invariably pairs with its complementary codon strategically lo-cated on an mRNA strand. The tRNA displayed specifically carries the amino acid ‘alanine’. The ‘anticodons’ are mostly represented and duly read in the 5′ → 3′ di-rection ; and, therefore, the anticodon for the amino acid ‘alanine’ may be read as C–G–A.

[Adapted From : Tortora GJ et al. : Microbiology : An Introduction,

The Benjamin and Cummings Publishing Co. Inc., New York, 5th edn., 1995]