Question

Do the following events during bacterial translation occur primarily within the \(30 \mathrm{~S}\) subunit, within the \(50 \mathrm{~S}\) subunit, or at the interface between these two ribosomal subunits? A. mRNA-tRNA recognition B. Peptidyl transfer reaction C. Exit of the polypeptide from the ribosome D. Binding of initiation factors IF 1, IF2, and IF3

Short answer

A. mRNA-tRNA recognition occurs in the 30S subunit. B. Peptidyl transfer reaction occurs in the 50S subunit. C. Exit of the polypeptide from the ribosome occurs in the 50S subunit. D. Binding of initiation factors IF1, IF2, and IF3 occurs in the 30S subunit.

Step-by-step solution

mRNA-tRNA recognition

mRNA-tRNA recognition occurs within the 30S subunit of the ribosome. The 30S subunit is where the initiator tRNA binds to the start codon of mRNA, facilitating the recognition process and ensuring the correct positioning of the mRNA sequence for translation.

Peptidyl transfer reaction

The peptidyl transfer reaction, which forms the peptide bond between amino acids during protein synthesis, occurs within the 50S subunit of the ribosome. This reaction is catalysed by the peptidyl transferase center (PTC), which is located in the 50S subunit.

Exit of the polypeptide from the ribosome

The exit of the synthesized polypeptide chain from the ribosome occurs through the exit tunnel present in the 50S subunit. As the polypeptide chain is synthesized it moves through this exit tunnel and emerges from the ribosome.

Binding of initiation factors IF 1, IF2, and IF3

The binding of the bacterial initiation factors IF1, IF2, and IF3, which facilitate the initiation of the translation process, occurs mainly within the 30S subunit of the ribosome. These factors help in positioning the mRNA and initiator tRNA correctly for the start of translation, and prevent premature binding of the 50S subunit.

Key concepts

Ribosome Subunits

The ribosome is the cellular machine responsible for protein synthesis, a process essential to all living organisms. In bacteria, the ribosome consists of two distinct subunits, the 30S and the 50S, each serving specific functions during translation.

The 30S subunit, a complex of RNA and proteins, plays a crucial role in decoding the genetic information from mRNA. It is here where mRNA-tRNA recognition takes place, ensuring the correct translation of the genetic code into proteins. The 50S subunit, larger than the 30S, is responsible for catalyzing the formation of peptide bonds—a key step known as the peptidyl transfer reaction. Together, these subunits form a complete ribosome, facilitating the orchestrated sequence of events that translates mRNA into amino acid sequences.

mRNA-tRNA Recognition

During the first crucial step of translation, mRNA-tRNA recognition must occur precisely to ensure the correct amino acid is added to the growing polypeptide chain. This process takes place within the 30S subunit of the ribosome and involves intricate interactions.

Transfer RNA (tRNA) molecules, each carrying a specific amino acid, contain an anticodon region that must pair accurately with the mRNA's codon. The 30S subunit houses the decoding center, where this base-pairing happens, establishing the genetic blueprint for the protein being synthesized. This specific recognition is vital, as even a small error can result in a faulty protein.

Peptidyl Transfer Reaction

Once the mRNA-tRNA pairing is confirmed, the ribosome proceeds with the peptidyl transfer reaction. Occurring within the 50S subunit, this reaction forms the core of the protein synthesis process.

The 50S subunit's peptidyl transferase center (PTC) is the catalyst for this critical chemical reaction. The PTC takes the amino acid from the incoming tRNA and forms a peptide bond with the growing polypeptide chain attached to the tRNA in the P site. This is a repetitive process, incrementally building the protein with each new correctly-matched tRNA that arrives.

Polypeptide Exit from Ribosome

As amino acids are linked together to form a polypeptide, the nascent protein must eventually exit the ribosome. This egress occurs through an exit tunnel located within the 50S subunit of the ribosome.

The tunnel guides the emerging polypeptide out of the ribosome's core to the outer cellular environment where it will fold into its functional three-dimensional structure. This tunnel is appropriately sized to accommodate unstructured polypeptide chains, but not larger, folded structures, ensuring that proteins do not assume their final shape until after they have been fully synthesized and released.

Binding of Initiation Factors

The initiation of translation is a highly regulated process that requires additional proteins known as initiation factors IF1, IF2, and IF3. These factors are critical for the proper assembly of the ribosome at the beginning of protein synthesis.

Binding of these initiation factors occurs mainly within the 30S subunit. They prepare the ribosome for translation by facilitating the binding of mRNA and the initiator tRNA that carries methionine, the first amino acid of most proteins. They also help to prevent the premature association of the 50S subunit until the mRNA and tRNA are properly positioned. This step is essential for the accuracy and efficiency of protein synthesis, as it sets the stage for the entire translation process.