Term
| Termination of translation |
|
Definition
A macromolecular machine comprised of at least 3 RNA molecules and
more than 50 proteins. Its composition is about half RNA and half protein.
Total mass is 2.5 mega Da.
Peptide bond synthesis occurs at ~ 20 amino acids/sec in prokaryotes or
~ 2-4 amino acids/sec in eukaryotes. For bacteria, this translation rate is at an
appropriate speed to keep pace with transcription (~50-100 nt/second).
Consequently, transcription and translation are often coupled in prokaryotes
(e.g. Rho dependent termination; attenuation). |
|
|
Term
| Energetics of protein synthesis |
|
Definition
One ATP molecule is required for charging of each aminoacyl-tRNA.
One GTP molecule is needed for EF-Tu -mediated delivery of a charged
tRNA to the A site of the ribosome.
Another GTP is consumed during the EF-G -mediated process of translocation.
Formation of each peptide bond requires one ATP and two GTP molecules.
(Additional GTP molecules are needed for initiation and termination of translation).
ATP provides energy for peptide bond formation, whereas GTP regulates
the accuracy and order of events in translation. |
|
|
Term
| Release factors terminate translation (Class I) |
|
Definition
Class I- recognize the stop codons and trigger hydrolysis of the peptide chain. Prokaryotes- RF1: recognizes UAG
RF2: recognizes UGA
Both RF1 and RF2 recognize UAA
Eukaryotes have a single eRF1 |
|
|
Term
| Release factors terminate translation (Class II) |
|
Definition
They stimulate the dissociation of Class I factors from the ribosome after release of the polypeptide chain.
There is only one factor in both prokaryotes (RF3) and eukaryotes (eRF3). |
|
|
Term
| Termination of translation |
|
Definition
Class I release factors recognize stop codons and induce peptide release.
Whether the GGQ motif in release factors directly promote hydrolysis
of the peptide bond, or whether it regulate the catalytic activity of rRNA
(which instead mediates hydrolysis) is not known. |
|
|
Term
| Termination of translation |
|
Definition
Class I RFs promote hydrolysis of the peptide bond.
Class II RFs, when bound to GDP, associate with
class I RFs on the ribosome.
After binding GTP, class II RFs stimulate dissociation
of class I RFs from the ribosome.
Interaction of class II RFs with the factor binding site
of the ribosome stimulates GTP hydrolysis.
In the absence of a bound class I RF,
the class II RF-GDP is released.
‘Masking’ of the factor binding site by the class I RF
ensures that the class II RF will remain bound to GTP
until it has done it’s ‘job’ of releasing the class I RF. |
|
|
Term
Release of tRNA and mRNA from ribosomes is
controlled by RRF, EF-G, and IF3 |
|
Definition
|
|
Term
| Many antibiotics inhibit steps in protein synthesis |
|
Definition
| In addition to being clinically important, many antibiotics have served as useful tools to study the various steps of protein synthesis. |
|
|
Term
| Puromycin causes premature termination of translation |
|
Definition
|
|
Term
Mechanisms to rescue ‘trapped’ ribosomes that
are stuck translating damaged mRNAs |
|
Definition
Incomplete transcription or degradation can produce mRNAs that lack a stop codon.
Such truncated, ‘damaged’ mRNAs pose two problems:
1. They can fail to release ribosomes, thereby ‘trapping’ these machines.
2. They can result in wasteful and possibly harmful production of aberrant proteins.
Both prokaryotes and eukaryotes have mechanisms to rescue trapped ribosomes
and prevent accumulation of aberrant mRNAs or proteins. |
|
|
Term
| The SsrA tmRNA rescues stalled ribosomes |
|
Definition
|
|
Term
Mechanisms of degradation of incomplete mRNAs/proteins
or rescue of stalled ribosomes in eukaryotes |
|
Definition
|
|
