Page 96 - Academic Press Encyclopedia of Physical Science and Technology 3rd Molecular Biology

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Encyclopedia of Physical Science and Technology EN017F-788 August 3, 2001 16:27

Translation of RNA to Protein 37

the synthetase reaction in attaching an amino acid to its The small subunit comprises a single rRNA of 0.3–
6
cognate tRNA is critically important to the fidelity of the 0.7 × 10 Da and single copies of 20 to 30 unique proteins.
translation process. Once the aminoacyl-tRNA has been It has a major function in binding initiator tRNA and
formed, the subsequent incorporation of the amino acid mRNA in the initiation of protein synthesis and in de-
residue into a polypeptide does not depend on the amino coding the genetic message.
acid itself but only on the interaction between the anti- The large subunit comprises a high-molecular-weight
6
codon of the aminoacyl-tRNA with the codon of mRNA. rRNA (0.6–1.7 × 10 Da) and often one or two smaller
6
Thus, an error in the synthetase reaction would lead to rRNAs (0.03–0.05 × 10 Da) and 30 to 50 different pro-
the incorporation of an inappropriate amino acid into the teins are present, with one exception, as single copies.
polypeptide. The large subunit binds aminoacyl-tRNA at the A-site,
Synthesis of aminoacyl-tRNA (III) from amino acids peptidyl-tRNA at the P site, and discharged tRNA at the
(I) requires activation of the amino acid carboxyl group E (exit) site. The large subunit contains the peptidyl trans-
with formation of an intermediate enzyme-bound amino- ferase and, unusually, this enzyme activity resides in the
acyladenylate (II). rRNA molecule itself rather than in the associated ribo-
somal proteins. This subunit is also involved in binding
H H elongation factor G which is required for translocation
(Fig. 10).
| |
−
E + R−CH−CO +ATP → E · R−CH−CO · AMP + PP i
2
| | ↓ B. The Ribosome Cycle in Translation
NH + NH +
3 3 2P i
Polypeptide synthesis can be divided into three stages:
(I) (II)
initiation, elongation, and termination (see Fig. 7). Initia-
H tion involves the binding of a ribosome to mRNA with the
| initiation codon correctly placed in the P-site. Elongation
→ R −CH−CO · tRNA + AMP + E leads to the stepwise increase in the length of the polypep-
| tide chain through the transfer of the growing chain to the
NH + amino group of aminoacyl-tRNA. Termination of chain
3
(III) elongation and release of the completed polypeptide oc-
curs when a termination codon reaches the A site. All
where E = aminoacyl-tRNA synthetase.
stages require the participation of protein factors. Ad-
The energy for the reaction (two high-energy phosphate
vances in establishing the structures of translational fac-
bonds) is provided by ATP and stored in the ester bond of
tors by X-ray crystallography, which have been rapid
the aminoacyl-tRNA to be used subsequently for peptide
in the last decade, are reviewed by Al-Karadaghi et al.
bond synthesis.
(2000).
IV. RIBOSOME STRUCTURE AND 1. Formation of Pre-Initiation Complexes
FUNCTION IN TRANSLATION
The ribosome cycle starts with the stepwise formation
of an initiation complex from mRNA, charged initiator
A. Ribosome Structure
tRNA, and ribosomal subunits. A number of pre-initiation
Ribosomes are high-molecular-weight complexes of RNA complexes are formed as intermediates and the process
(rRNA) and proteins (Table I), and the electron-dense par- is facilitated by initiation factors. In outline, prokaryotic
ticles are easily visualized by electron microscopy. Ri- and eukaryotic systems are similar, but there are a few
bosomes from various sources (prokaryotes, eukaryotic differences, particularly as regards the complexity of the
cytoplasm, mitochondria, chloroplasts, and kinetoplasts) initiation factors and details of the mechanisms.
vary in size from 20 to 30 nm in diameter, but all are com-
posed of a large and a small subparticle or subunit and a. Prokaryotic systems. Three proteins, initiation
perform similar functions in protein synthesis. The prin- factors IF-1, IF-2, and IF-3 (see Table II), are required
cipal functional domains of the ribosome and associated for the initiation of protein biosynthesis (see Fig. 8a).
components are given in Fig. 10. More detailed resolution Ribosomal subunits are released by dissociation of ribo-
of the ribosome structure has allowed the placement of somes following translation of the mRNA. Dissociation is
mRNA,aminoacyl-tRNA,peptidyl-tRNA,andthenascent facilitated by the combined action of the initiation factors
polypeptide chain (see Section C and Fig. 11). IF-1 and IF-3; IF-1 increases the rate of dissociation and