How Many GTP Molecules Are Required for tRNA Translocation from P Site to E Site in Bacteria?

Understanding tRNA Translocation During Bacterial Translation

The correct answer is 1 GTP molecule. During the elongation phase of bacterial protein synthesis, the ribosome repeatedly moves along the messenger RNA (mRNA) by one codon at a time. This movement is called translocation. The process is mediated by the bacterial elongation factor EF-G, which binds to the ribosome in its GTP-bound form and is associated with the hydrolysis of one molecule of GTP during each translocation event.

After peptide bond formation, the ribosome contains a peptidyl-tRNA associated with the A site and a deacylated, or uncharged, tRNA associated with the P site. The ribosome must now move forward by exactly one codon so that translation can continue. During this movement, the peptidyl-tRNA shifts from the A site to the P site, while the deacylated tRNA shifts from the P site to the E site.

The E site is called the exit site because the uncharged tRNA subsequently leaves the ribosome from this position. This coordinated movement is promoted by EF-G, also known as translocase. Therefore, the movement of tRNA from the P site to the E site during one elongation cycle requires one GTP molecule.

What Happens Before the Translocation Step?

Before translocation occurs, a new aminoacyl-tRNA enters the A site of the bacterial ribosome. The correct aminoacyl-tRNA is delivered by another elongation factor called EF-Tu. EF-Tu also uses one GTP molecule, but this GTP is consumed during the delivery and selection of aminoacyl-tRNA at the A site, not during the translocation of tRNA from the P site to the E site.

Once the correct aminoacyl-tRNA is positioned in the A site, the ribosome catalyzes peptide bond formation. The growing polypeptide chain is transferred from the tRNA in the P site to the amino acid attached to the tRNA in the A site. This reaction is catalyzed by the peptidyl transferase center of the large ribosomal subunit.

After peptide bond formation, the ribosome enters a pre-translocation state. The tRNA in the P site becomes deacylated, while the tRNA in the A site carries the growing polypeptide chain. At this point, EF-G binds to the ribosome and promotes the translocation step.

Role of EF-G and GTP in Ribosomal Translocation

EF-G Acts as the Bacterial Translocase

EF-G is a GTP-binding elongation factor that plays a central role in bacterial translation. It interacts with the ribosome after peptide bond formation and promotes the coordinated movement of the ribosome, mRNA and tRNAs.

When EF-G binds to the ribosome, it carries one molecule of GTP. GTP hydrolysis and the associated conformational changes in EF-G and the ribosome promote forward translocation. As a result, the ribosome advances by one codon along the mRNA.

During this process, the peptidyl-tRNA moves from the A site to the P site, while the deacylated tRNA moves from the P site to the E site. The uncharged tRNA can then leave the ribosome, and the A site becomes empty and ready to accept the next aminoacyl-tRNA. Thus, one EF-G-mediated translocation event requires one GTP molecule.

Movement of tRNA Through the A, P and E Sites

A Site: Aminoacyl Site

The A site is the entry site for an incoming aminoacyl-tRNA. A charged tRNA carrying the correct amino acid enters this site according to the codon present on the mRNA. In bacteria, this delivery is mediated by EF-Tu and requires the utilization of one GTP molecule.

P Site: Peptidyl Site

The P site holds the tRNA associated with the growing polypeptide chain before peptide bond formation. After the peptide bond is formed, the growing polypeptide chain is transferred to the tRNA located in the A site. The tRNA remaining in the P site becomes uncharged.

E Site: Exit Site

The E site is the exit site of the ribosome. During EF-G-mediated translocation, the deacylated tRNA moves from the P site to the E site and subsequently leaves the ribosome. This movement is part of the overall translocation event that requires one GTP molecule.

Why Is 1 GTP Molecule the Correct Answer?

The question specifically asks about the translocation of tRNA from the P site to the E site during bacterial translation elongation. This movement occurs as part of a single ribosomal translocation event. The elongation factor EF-G participates in this step and utilizes one GTP molecule.

It is important to distinguish this translocation-associated GTP from the GTP used during aminoacyl-tRNA delivery. EF-Tu uses one GTP to deliver the correct aminoacyl-tRNA to the A site, whereas EF-G uses another GTP for ribosomal translocation. Therefore, although two GTP molecules are generally consumed during one complete elongation cycle, only one GTP molecule is associated with the translocation step itself.

Explanation of All Possible Answers

Why 0 GTP Is Incorrect

The answer cannot be zero because bacterial ribosomal translocation is mediated by EF-G, a GTP-binding protein. EF-G binds to the ribosome and promotes the conformational changes required for efficient forward translocation. Therefore, no GTP is not the correct answer.

Why 1 GTP Is Correct

One GTP is the correct answer because a single EF-G-mediated translocation event moves the ribosome by one codon. During this event, the peptidyl-tRNA moves from the A site to the P site, while the deacylated tRNA moves from the P site to the E site. Therefore, the specific P-site-to-E-site movement requires one GTP molecule.

Why 2 GTP Is Incorrect

Two GTP molecules are commonly associated with one complete elongation cycle, but they are used in two different steps. One GTP is used by EF-Tu during aminoacyl-tRNA delivery to the A site, while the second GTP is used by EF-G during translocation. Since the question asks specifically about translocation from the P site to the E site, only the EF-G-associated GTP should be counted.

Why More Than 2 GTP Molecules Is Incorrect

More than two GTP molecules are not required for a single standard bacterial elongation cycle. The two major GTP-consuming events are aminoacyl-tRNA delivery by EF-Tu and translocation by EF-G. The specific movement of tRNA from the P site to the E site is part of one translocation event and therefore requires only one GTP molecule.

Energy Requirement of One Bacterial Translation Elongation Cycle

For each amino acid added to a growing polypeptide chain, the bacterial translation machinery uses energy at several stages. Aminoacyl-tRNA formation requires ATP, while the elongation phase uses GTP.

One GTP molecule is used when EF-Tu delivers the correct aminoacyl-tRNA to the A site. A second GTP molecule is used during EF-G-mediated translocation. Therefore, a complete elongation cycle generally consumes two GTP molecules, but the translocation step alone consumes one GTP molecule.

This distinction is essential for correctly answering questions that ask specifically about individual stages of bacterial protein synthesis.

Final Answer

Correct Answer: 1 GTP molecule

During bacterial translation elongation, the elongation factor EF-G promotes ribosomal translocation. In this process, the peptidyl-tRNA moves from the A site to the P site, while the deacylated tRNA moves from the P site to the E site. A single translocation event requires one GTP molecule. Therefore, the translocation of tRNA from the P site to the E site requires 1 GTP molecule.