TRNA function

A large number of components are required for the synthesis of a polypeptide chain. These include all the amino acids that are found in the finished product, the mRNA to be translated, tRNAs, functional ribosomes, energy sources, and enzymes, as well as protein factors needed for initiation, elongation, and termination of the polypeptide chain. 

Some data suggested that a transient covalent linkage of tRNA to the synthetases may form through addition of a nucleophilic group of the enzyme to the 6 position of the uracil (or 4-thiouracil) present in position 8 of all tRNAs (Eq. 19-3).257 The two isoacceptors tRNATyr species in E. coli contain 4-thiouracil at this position. The C=C bond in this base can be saturated by sodium borohydride reduction, which was found not only to prevent the covalent interaction with the enzyme but also to prevent aminoacylation of the tRNA. However, Eq. 29-3 probably describes a side reaction irrelevant to tRNA function. 

The CCA sequence is conserved at the tRNA 3 end as a single stranded motif with a wide range of dynamic conformations that are essential for tRNA functions. The CCA sequence is synthesized and... 

R. Basavappa and P.B. Sigler. 1991. The 3 A crystal structure of yeast initiator tRNA Functional implications in initiator/elongator discrimination MBO J. 10 3105-3111. (PubMed)... 

In eukaryotes, the initiator aminoacyl-tRNA is not formylated but instead is a unique tRNA functioning only in initiation as Met-tRNAi . Eukaryotic initiation requires a set of eukaryotic initiation factors (elFs), as listed in Table 13.9, and proceeds in three steps ... 

II. PROPOSED MODELS FOR tRNA FUNCTION IN REGULATION A. Modulation Hypothesis... 

Yet, studies in bacteria continue to yield intriguing results when E. coli are grown in a methionine-deficient medium, tRNA is synthesized but not methylated. Such tRNA usually binds to codon enzymes and ribosomes normally. However, there are exceptions and further studies are needed to evaluate the role of methylation in tRNA function. 

Bridges are found not only between the two subunits but also between the subimits and tRNAs. The tRNAs function is to orient the subunits and facilitate movement. They also may function in signaling between the subunits and coordinating the multiple steps in the cycle of peptide elongation. 

By relating individual codons of mRNA to the cognate amino acids, tRNA functions as a key bilingual intermediate in the translation of the genetic code. All transfer RNAs are single-stranded molecules about 80 nucleotides long with a common 3 -terminal CCA sequence. Most of the bases are standard but some (e.g., pseudoU, dihydroU, and T) are derived by modification after transcription of the transfer RNA genes. 

The aminoacyl domain of tRNA composed of the CCA end, seven base pairs of the aminoacyl stem and five base pairs of the T stem, is sufficient to promote efficient binding to the EF-Tu GTP [68]. About 10 base pairs of the aminoacyl domain are bound to domain III of EF-Tu [79]. This binding is probably governed by ionic interactions of the RNA-phos-phate backbone. The sequence of nucleotides, however, also plays some role in this process. EF-Tu GTP serves for aminoacyl-tRNA not only as a vehicle transporting it to the ribosome but in addition as a matrix setting the correct conformation of the L shaped molecule [80]. The precise distance between the anticodon loop and aminoacyl-residue is evidently required for aminoacyl-tRNA functioning during translation [50]. 

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