Transfer RNA (tRNA) is a small RNA molecule that plays a central role in protein synthesis. It functions as a molecular adapter, bridging genetic information encoded in messenger RNA (mRNA) with the specific amino acids that form proteins. tRNA decodes mRNA codons and delivers the corresponding amino acids, ensuring precise protein construction.
tRNA Production Sites
The creation of tRNA molecules occurs in different cellular locations depending on the organism. In eukaryotic cells, tRNA is transcribed from DNA within the nucleus by RNA Polymerase III, which synthesizes precursor tRNA (pre-tRNA) molecules.
Following transcription, these pre-tRNA molecules undergo several processing steps in the nucleus. These steps include trimming extra nucleotides from their 5′ and 3′ ends, adding a CCA sequence to the 3′ end, and in some cases, removing introns through splicing. Chemical modifications to specific bases also occur, contributing to the mature tRNA’s structure and function. In contrast, prokaryotic cells, which lack a nucleus, synthesize their tRNA molecules directly in the cytoplasm, where transcription and processing often happen concurrently.
tRNA’s Role at Ribosomes
Most functional tRNA molecules reside and perform their primary function in the cell’s cytoplasm. This cytoplasmic presence is important because protein synthesis, known as translation, takes place on ribosomes in this cellular compartment. Ribosomes, which can be free in the cytoplasm or attached to the endoplasmic reticulum, serve as the machinery where mRNA is translated into protein.
During translation, tRNA molecules associate with these ribosomes. Each ribosome contains three binding sites: the A (aminoacyl) site, the P (peptidyl) site, and the E (exit) site. An incoming tRNA with a new amino acid enters the A site, matching its anticodon to the mRNA codon. The P site holds the tRNA attached to the growing polypeptide chain, while the E site is where the uncharged tRNA exits for recycling. This movement allows tRNA to deliver correct amino acids, forming the polypeptide chain.
Moving tRNA Across Cell Compartments
In eukaryotic cells, tRNA moves between different cellular compartments. Once synthesized and processed in the nucleus, mature tRNA molecules are actively transported out of the nucleus into the cytoplasm. This regulated transport occurs through nuclear pore complexes, which are gateways embedded in the nuclear membrane.
A transport protein, Exportin-t (also known as XPO3), plays a role in this nuclear export. Exportin-t binds to mature tRNA molecules in the nucleus and, in conjunction with the Ran-GTPase cycle, facilitates their passage into the cytoplasm. Within the cytoplasm, tRNA molecules are constantly recycled: recruited to ribosomes, releasing their amino acids, and becoming available to pick up another amino acid for subsequent rounds of translation. In prokaryotic cells, complex compartmental transport is not necessary, as both tRNA synthesis and its function occur within the single cytoplasmic compartment.