Processing of tRNA and rRNA

ZOOHCC - 501: Molecular Biology (Theory)

Unit 4: Post Transcriptional Modifications, Processing of Eukaryotic RNA and Gene Regulation

Processing of tRNA 

The processing of transfer RNA (tRNA) involves several steps that are necessary for the production of functional tRNA molecules. tRNAs are synthesized as precursor molecules that require several modifications to form mature tRNAs. The processing steps include:

Transcription: The DNA sequence that encodes tRNA is transcribed into a primary transcript by RNA polymerase III.

5' end processing: The primary transcript of tRNA is cleaved by an endonuclease to remove the 5' leader sequence, which is not part of the mature tRNA.

3' end processing: The primary transcript of tRNA is cleaved by an endonuclease to remove the 3' trailer sequence. A CCA sequence is then added to the 3' end of the tRNA by the enzyme tRNA nucleotidyltransferase.

Base modification: Specific bases in the tRNA molecule are modified enzymatically to produce mature tRNA. These modifications include the conversion of uridine to pseudouridine, the methylation of adenosine, cytosine, and guanine, and the thiolation of uridine.

Splicing: In some cases, the primary transcript of tRNA contains introns that must be removed by splicing. This process is carried out by endonucleases and ligases to produce mature tRNA.

Quality control: The processed tRNA molecules are subjected to a quality control mechanism to ensure that they are functional and properly folded. Misfolded or defective tRNA molecules are targeted for degradation by the cell.

These processing steps are essential for the production of functional tRNA molecules, which play a critical role in protein synthesis by transferring amino acids to the ribosome during translation.

rRNA and tRNA are structural molecules that aid in protein synthesis, but are not themselves translated into proteins.

• Ribosomal RNA (rRNA) is the structural molecule that makes up more than half the mass of the ribosome and aids in protein synthesis.

• Transfer RNA (tRNA) recognizes codons on the mRNA and places the appropriate amino acid in that position.

• rRNA is processed from the large pre-rRNA by cleaving the large rRNA and methylating some of its nucleotides.

• tRNAs are processed from pre-tRNAs by truncating both ends of the pre-tRNA, optionally adding CCA trinucleotides to the 3' ends, removing existing introns, and chemically modifying an average of 12 nucleotides per tRNA. increase.

Processing of tRNAs and rRNAs

tRNA and rRNA are structural molecules involved in protein synthesis. However, these RNAs themselves are not translated. In eukaryotes, rRNA precursors are transcribed, processed, and assembled into ribosomes in the nucleolus, whereas pre-tRNAs are transcribed and processed in the nucleus and released into the cytoplasm, where they bind free amino acids for protein synthesis. will be

Ribosomal RNA (rRNA)

The four eukaryotic rRNAs are first transcribed as two long precursor molecules. One contains only pre-rRNA and is processed into 5S rRNA. The other is 28S, 5.8S, and 18S rRNA. The enzyme then cleaves the precursor into subunits corresponding to each rRNA. In bacteria, there are only three rRNAs, all transcribed into long precursor molecules that are cleaved into individual rRNAs. Some bases in pre-rRNA are methylated to increase stability. Mature rRNA constitutes 50-60% of each ribosome. Some ribosomal RNA molecules are purely structural, while others have catalytic or binding activity. Eukaryotic ribosomes are composed of two subunits, the large subunit (60S) and the small subunit (40S). The 60S subunit is composed of 28S rRNA, 5.8S rRNA, 5S rRNA and 50 proteins. The 40S subunit is composed of 18S rRNA and 33 proteins. Bacterial ribosomes consist of two similar subunits that differ slightly in composition. The large bacterial subunit is called the 50S subunit and consists of 23S rRNA, 5S rRNA and 31 proteins, and the small bacterial subunit is called the 30S subunit and consists of 16S rRNA and 21 proteins. Two subunits combine to form a functional ribosome capable of making proteins.

Transfer RNA (tRNA)

Each tRNA binds to a specific amino acid and transfers it to the ribosome. The mature tRNA adopts a three-dimensional structure through intramolecular base pairing, placing an amino acid binding site at one end and an anticodon in an unpaired nucleotide loop at the other end. Anticodons are three-nucleotide sequences unique to each tRNA that interact with messenger RNA (mRNA) codons through complementary base pairing.

 There is a different tRNA for every 21 amino acids. Most amino acids can be carried by multiple tRNAs.

In all organisms, tRNA is transcribed in a pre-tRNA form that requires several processing steps before the mature tRNA is available for translation. In bacteria, multiple tRNAs are often transcribed as a single RNA. The first step in their processing is to digest the RNA to release individual pre-tRNAs. In archaea and eukaryotes, each pre-tRNA is transcribed as a separate transcript.

• The process of converting pre-tRNA to mature tRNA involves five steps.
• The 5' end of the pre-tRNA, the so-called 5' leader sequence, is cleaved.
• The 3' end of the pre-tRNA is cleaved.
• In all eukaryotic pre-tRNAs, a CCA nucleotide sequence is added to the 3' end of the pre-tRNA after truncating the original 3' end, but some bacterial and archaeal pre-tRNAs only included. Some bacterial and archaeal pre-tRNAs already have a CCA encoded just before the 3' cleavage site of the transcript, so no addition is necessary. The CCA at the 3' end of the mature tRNA is where amino acids are added to the tRNA.
• Some nucleotides of the pre-tRNA are chemically modified to change the nitrogenous base. On average, approximately 12 nucleotides are modified per tRNA. The most common modifications are the conversion of adenine (A) to pseudouridine (ψ), adenine to inosine (I), and uridine to dihydrouridine (D). However, over 100 other changes can occur.

A significant number of eukaryotic and archaeal pre-tRNAs have introns that require splicing. Introns are rare in bacterial pre-tRNAs, but are occasionally present and spliced ​​out. After processing, the mature pre-tRNA is ready to bind its cognate amino acid. A tRNA's cognate amino acid is that specified by its anticodon. This amino acid addition is called tRNA charging. In eukaryotes, mature tRNAs are produced in the nucleus and transported to the cytoplasm for loading.