In eukaryotic cells, tRNA is synthesized through a process involving transcription by RNA polymerase III, followed by processing of the pre-tRNA.
Steps in tRNA Synthesis
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Transcription: A special protein, RNA polymerase III, reads the DNA code and creates an RNA copy called pre-tRNA. This occurs within the nucleus. RNA polymerase III specifically recognizes promoters associated with tRNA genes.
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Processing: The pre-tRNA molecule undergoes several processing steps after it exits the nucleus to become a functional tRNA:
- 5' Leader Removal: An enzyme called RNase P removes a sequence of nucleotides from the 5' end of the pre-tRNA.
- 3' Trailer Removal and CCA Addition: A trailer sequence at the 3' end is removed, and the sequence CCA is added to the 3' end. This CCA sequence is crucial for tRNA's function in protein synthesis, as it's where the amino acid will attach.
- Intron Splicing (if present): Some pre-tRNAs contain introns, which are non-coding sequences that need to be removed by splicing.
- Base Modification: Many of the bases within the tRNA molecule are chemically modified. These modifications are important for the tRNA's structure, stability, and ability to interact with ribosomes and mRNA. Examples include methylation, deamination, and reduction.
Enzymes Involved
- RNA Polymerase III: Transcribes tRNA genes.
- RNase P: Removes the 5' leader sequence.
- tRNA nucleotidyltransferase: Adds the CCA sequence to the 3' end.
- Splicing enzymes: Remove introns (if present).
- Modifying enzymes: Carry out base modifications.
The fully processed tRNA molecule is then ready to participate in protein synthesis by delivering the correct amino acid to the ribosome based on the mRNA codon sequence.
