Translation in Prokaryotes

Translation in prokaryotes, the process of protein synthesis from mRNA, involves several key steps: initiation, elongation, and termination. Each step is facilitated by various ribosomal subunits, factors, and RNA molecules.

1. Initiation: The initiation step is considered the primary step or the first step of the translation process, where all the machinery required for protein synthesis is assembled. This assembly of components is completed in two processes, which are described below: 

Formation of the Initiation Complex (FIC): FIC is the first step in translation, where the mRNA and ribosome are assembled. The details of how this occurs are described below: 

1. • mRNA Binding (see in the above figure): The small ribosomal subunit (30S) attaches to the mRNA. It starts at a special spot on the mRNA called the Shine-Dalgarno sequence, which is just before the start codon (AUG). The sequence of the Shine-Dalgarno region is complementary to a sequence in the 16S rRNA of the ribosome, which helps them pair up correctly and align the mRNA with the ribosome. 

But it's also important to understand what factors are essential for their binding. This is where the role of Initiation Factors (IFs) comes in, which are defined as the proteins that play a crucial role in the beginning phase of translation, the process by which proteins are synthesized from mRNA. There are different IFs that have different roles and are described below. 

Role and Function of Initiation Factors

Initiation factors facilitate various steps of the initiation process, such as the binding of the ribosome to mRNA, the selection of the start codon, and the assembly of the complete ribosome with the initiator tRNA.

Key Initiation Factors in Prokaryote

 a. IF-1 (Initiation Factor 1):

1. • Function: Binds to the A site of the small ribosomal subunit (30S) to prevent premature entry of tRNAs, ensuring that the correct initiator tRNA binds to the P site.

   • Role: It stabilizes the initiation complex and prevents incorrect binding of aminoacyl-tRNAs.

 b. IF-2 (Initiation Factor 2):

1. • Function: A GTP-binding protein that facilitates the binding of the initiator tRNA (fMet-tRNA) to the P site of the small ribosomal subunit.

   • Role: It ensures that the initiator tRNA is correctly placed at the start codon. Upon binding to the small ribosomal subunit and the initiator tRNA, it hydrolyzes GTP to provide the energy required for the assembly of the large ribosomal subunit (50S).

 c. IF-3 (Initiation Factor 3):

1. • Function: Binds to the small ribosomal subunit (30S) to prevent it from prematurely associating with the large ribosomal subunit (50S).

   • Role: Ensures that the mRNA and the initiator tRNA are correctly positioned on the 30S subunit before the large subunit joins, aiding in the proper selection of the start codon and promoting the dissociation of the 70S ribosome into its subunits to initiate a new round of translation.

Binding of tRNA: tRNA Binding: The initiator tRNA carrying formylmethionine (fMet-tRNA) binds to the P site of the 30S subunit. This tRNA recognizes the start codon AUG.

Assembly of the Complete Ribosome: The 50S large ribosomal subunit joins the 30S subunit to form the functional 70S ribosome, displacing the initiation factors. The mRNA, the initiator tRNA (fMet-tRNA), and the ribosome are now ready for the elongation phase.