Question

What are the three initiation factors used by the bacteria to enter the initiation complex?

Answer 1

All bacteria need initiation factors for the translation of tRNA and mRNA. Translation is one of the stages of protein synthesis in which messenger mRNA acts as a template for synthesis. It involves four stages- initiation, elongation, termination and ribosome recycling.

Complete answer:
Protein synthesis takes place after the formation of protein synthesis. Bacteria make use of three initiation factors required for tRNA and mRNA to enter the initiation complex. The factors are IF-1, IF-2 and IF-3. Let us discuss these factors:
1. IF-1: It is the smallest of all the initiation factors with molecular mass of 8.2kDa encoding by the infA gene. It is only associated with 30S subunits as a portion of the complete initiation complex. It associates with the A site preventing entry of aminoacyl-tRNA. The binding of 30S subunit with 50S subunit can be hindered by its location. IF1 belongs to the family of oligonucleotide binding fold proteins. It consists of 5 strand beta barrels with a loop in between the strand 3 and 4, capping one end of the barrel. Several functions have been attributed to bacterial IF1 as it enhances the dissociation and association rate for 70 S ribosomes. It is essential for cell viability and many of its functions are essential.
2. IF-2: It is the largest initiation factor that is encoded by the infB gene. It is associated with a special initiator tRNA regulating their entry to the ribosome. They are found in archaebacteria and eubacteria. The main function of them is GTPase activity, promotion of ribosomal subunit association and interaction with the initiator tRNA. It is one of the three initiation factors that displays a relative high and specific affinity for ribosomal units. Its structure is complex with the ribosome. The expression of IF2 is upregulated during shock to cold and it forms up a stable complex unfolding proteins. It is used for classification of organisms that are closely related.
3. IF-3: It is composed of 180 amino acids that encodes for the essential infC gene. These gene are transcribed from 4 promoters and terminated by 2 transcriptional terminators. It is required for 30S subunits to specifically associate to the initiation sites in the mRNA. It performs several functions like preventing association of the ribosomal subunits by binding to the 30S units and block 50S units, monitors codon- anticodon interaction by promoting dissociation of complexes. It interacts with the anticodon stem and with the loop of fMet- tRNA. It is unable to reach three conserved discriminator GC base pairs that are present in the anticodon stem. Hence, discrimination agaisnt tRNA is promoted by IF3 indirectly.

Note:
IF1 and IF2 are universally present and they are needed for the function of all living cells whereas IF3 is limited to a number of bacterial species and they are usually found in some plastids. These bacterias have the simplest machinery for translation machinery.