Biology
- Amino Acid Activation
Codons of an mRNA molecule contain genetic messages that are carried by the mRNA and they need to be translated to form the corresponding sequence of amino acids that will form the polypeptide chain and subsequently the protein. The tRNA transfers amino...
- Trnas Are Charged With Amino Acids By Synthetases
KEY TERMS:Cognate tRNAs (Isoaccepting tRNA) are those recognized by a particular aminoacyl-tRNA synthetase. They all are charged with the same amino acid. KEY CONCEPTS: Aminoacyl-tRNA synthetases are enzymes that charge tRNA with an amino acid to...
- Mrna Degradation Involves Multiple Activities
KEY CONCEPTS:Degradation of yeast mRNA requires removal of the 5 cap and the 3 poly(A). One yeast pathway involves exonucleolytic degradation from 5?3. Another yeast pathway uses a complex of several exonucleases that work in the 3?5 direction. The deadenylase...
- Bacterial Mrna Degradation Involves Multiple Enzymes
KEY TERMS:The degradosome is a complex of bacterial enzymes, including RNAase and helicase activities, which may be involved in degrading mRNA. KEY CONCEPTS: The overall direction of degradation of bacterial mRNA is 5?3. Degradation results from the...
- Transfer Rna (trna) Forms A Cloverleaf
KEY TERMS:Transfer RNA (tRNA) is the intermediate in protein synthesis that interprets the genetic code. Each tRNA can be linked to an amino acid. The tRNA has an anticodon sequence that complementary to a triplet codon representing the amino acid....
Biology
tRNAs are processed from longer precursors
KEY CONCEPTS:
- A mature tRNA is generated by processing a precursor.
- The 5
end is generated by cleavage by the endonuclease RNAase P.
- The 3
end is generated by cleavage followed by trimming of the last few bases, followed by addition of the common terminal trinucleotide sequence CCA.
tRNAs are commonly synthesized as precursor chains with additional material at one or both ends. Figure 7.6 shows that the extra sequences are removed by combinations of endonucleolytic and exonucleolytic activities. One feature that is common to all tRNAs is that the three nucleotides at the 3![]()
terminus, always the triplet sequence CCA, are not coded in the genome, but are added as part of tRNA processing (for review see Hopper and Phizicky, 2003).
The 5![]()
end of tRNA is generated by a cleavage action catalyzed by the enzyme ribonuclease P.
The enzymes that process the 3![]()
end are best characterized in E. coli, where an endonuclease triggers the reaction by cleaving the precursor downstream, and several exonucleases then trim the end by degradation in the 3![]()
-5![]()
direction. The reaction also involves several enzymes in eukaryotes. It generates a tRNA that needs the CCA trinucleotide sequence to be added to the 3![]()
end.
The addition of CCA is the result solely of an enzymatic process, that is, the enzymatic activity carries the specificity for the sequence of the trinucleotide, which is not determined by a template. There are several models for the process, which may be different in different organisms.
In some organisms, the process is catalyzed by a single enzyme. One model for its action proposes that a single enzyme binds to the 3![]()
end, and sequentially adds C, C, and A, the specificity at each stage being determined by the structure of the 3![]()
end. Other models propose that the enzyme has different active sites for CTP and ATP.
In other organisms, different enzymes are responsible for adding the C and A residues, and they function sequentially.
When a tRNA is not properly processed, it attracts the attention of a quality control system that degrades it. This ensures that the protein synthesis apparatus does not become blocked by nonfunctional tRNAs.
- Amino Acid Activation
Codons of an mRNA molecule contain genetic messages that are carried by the mRNA and they need to be translated to form the corresponding sequence of amino acids that will form the polypeptide chain and subsequently the protein. The tRNA transfers amino...
- Trnas Are Charged With Amino Acids By Synthetases
KEY TERMS:Cognate tRNAs (Isoaccepting tRNA) are those recognized by a particular aminoacyl-tRNA synthetase. They all are charged with the same amino acid. KEY CONCEPTS: Aminoacyl-tRNA synthetases are enzymes that charge tRNA with an amino acid to...
- Mrna Degradation Involves Multiple Activities
KEY CONCEPTS:Degradation of yeast mRNA requires removal of the 5 cap and the 3 poly(A). One yeast pathway involves exonucleolytic degradation from 5?3. Another yeast pathway uses a complex of several exonucleases that work in the 3?5 direction. The deadenylase...
- Bacterial Mrna Degradation Involves Multiple Enzymes
KEY TERMS:The degradosome is a complex of bacterial enzymes, including RNAase and helicase activities, which may be involved in degrading mRNA. KEY CONCEPTS: The overall direction of degradation of bacterial mRNA is 5?3. Degradation results from the...
- Transfer Rna (trna) Forms A Cloverleaf
KEY TERMS:Transfer RNA (tRNA) is the intermediate in protein synthesis that interprets the genetic code. Each tRNA can be linked to an amino acid. The tRNA has an anticodon sequence that complementary to a triplet codon representing the amino acid....