Uncharged tRNA

Figure 20.25 Regulation of the activities of the aminoacyl-tRNA synthetases by the concentrations of free tRNAs (i.e. uncharged tRNA). Changes in the concentrations of free tRNAs provide the mechanism for communication between control via the initiation factor (Figure 20.20) and ribosomal protein kinase (steps 6 and 7) and the flux-generating step.

In the 70 S initiation complex, formylme-thionine tRNA is initially located at a binding site known as the peptidyl site (P). A second binding site, the acceptor site (A), is not yet occupied during this phase of translation. Sometimes, a third tRNA binding site is defined as an exit site (E), from which uncharged tRNAs leave the ribosome again (see p. 252 not shown). 

The ribosome then translocates to the next codon, with the peptidyl-tRNA shifting from the A site to the P site and the now uncharged tRNA exiting the ribosome from the E site. 

The elongation cycle in eukaryotes is quite similar to that in prokaryotes. Three eukaryotic elongation factors (eEFla, eEFljSy, and eEF2) have functions analogous to those of the bacterial elongation factors (EF-Tu, EF-Ts, and EF-G, respectively). Eukaryotic ribosomes do not have an E site uncharged tRNAs are expelled directly from the P site. 

Generation of the initiator N-formyl-> movement of the uncharged tRNA into the ribosomal E site (before... 

One experimental point worth noting in regard to Figure 7.8 is that the rate constant for the deacylation of the mischarged tRNA can be measured by the pre-steady state kinetics even in the presence of a large fraction of uncharged tRNA. This is not easily done by steady state kinetics, because of the competi-... 

Schematic diagram of ppGpp synthesis and the hypothesized mechanism for its action. ppGpp is synthesized on the ribosome when there is a peptidyl-tRNA on the P site of the ribosome and uncharged tRNA on the A site. The ppGpp probably inhibits rRNA synthesis by complexing with the RNA polymerase.

Translocation, however, requires the input of energy (again, in the form of GTP) with the participation of the elongation factor EF-G. The translocation reaction moves the peptidyl-tRNA from the A-site to the P-site. The uncharged tRNA is removed from the P-site (it remains bound at an Exit or E-site for another cycle of elongation), while the ribosome and mRNA move relative to each other. This is shown in Figure 11-8. 

In several bacterial species, uncharged tRNA serves as the effector molecule in controlling expression of several aminoacyl-tRNA synthetase genes and a few amino acid biosynthetic oper-ons by a conunon mechanism termed T-box antitermination. 

When an amino acid is covalently linked to a tRNA molecule, the tRNA is said to be aminoacylated or charged. The notation for a tRNA charged with serine is seryl tRNA. The term uncharged tRNA refers to a tRNA molecule lacking an amino acid. 

Hao S, Sharp JW, Ross-Inta CM, McDaniel BJ, Anthony TG, et al. 2005a. Uncharged tRNA and sensing of amino acid deficiency in mammalian piriform cortex. Science 307 1776-1778. (First report of the full biochemical pathway from a measured decrease in the limiting lAA to phosphorylation of elF2a in the brain area that serves as the sensor for lAA depletion)... 

Wek SA, Zhu S, Wek RC. 1995. The histidyl-tRNA synthetase-related sequence in the eJF-2a protein kinase GCN2 interacts with tRNA and is required for activation in response to starvation for different amino acids. Mol Cell Biol 15 4497-4506. (Demonstrates the mechanism of transduction of the uncharged tRNA signal by the eIF2a kinase, GCN2)... 

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