Page 102 - 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 43

artificial cell-free systems, any N-substituted aminoacyl- TABLE V Eukaryotic Elongation and Termination Factors
tRNA, such as peptidyl-tRNA or N-acetylaminoacyl- M r (kDa) Properties and function
tRNA, can function in peptide bond synthesis as a donor
in the P site in place of the charged initiator tRNA. The Elongation factors from various yeast, animal, and plant cells
reaction is catalyzed by the peptidyltransferase activity of eEF-1A (EF-1 L 50–60 Analogous to EF-T u
the large ribosomal subunit. No soluble cofactors appear or eEF-T u )
+
to be involved, but monovalent cations (K ) at a concen- eEF-1B (eEF-T s ) 30 Analogous to EF-T s
2+
tration of 100 mM or more and divalent cations (Mg ) eEF-2 105 Contains essential SH groups and
one residue of a post-translationally
below 2 mM are required.
modified histidine residue;
Efficient entry of aminoacyl-tRNA into the ribosomal GTP-dependent translocation
A site requires the participation of an elongation factor, analogous to EF-G
termed EF-Tu in prokaryotes (see Table IV) and EF-1 eEF-3 125 GTPase and ATPase activity; function
(EF-1 L ) in eukaryotes (see Table V), and GTP. This elon- not fully defined
gation factor forms a ternary complex with GTP and all Termination or release factors
aminoacyl-tRNAs except initiator tRNA, but not with eRF-1 110 Two 55-kDa subunits; binds to the
uncharged tRNA, thus ensuring that only appropriately ribosome A site by a GTP and
termination codon dependent
charged tRNAs are efficiently bound in the A site. A spe-
reaction; hydrolyzes peptidyl-tRNA
cial elongation factor showing extensive homology with in the P site
both EF-Tu and IF-2 is involved in the synthesis of seleno- eRF-2 GTPase; stimulates eRF-1 activity
proteins (see Section II.C) from selenocysteyl-tRNA UCA
in E. coli.
The above-mentioned aminoacyl-tRNA binding reac- formation of the peptide bond. The elongation factor is
tion catalyzed by EF-Tu is the rate-limiting step in the later released from the ribosome as a complex with GDP.
elongation cycle; peptide bond formation and transloca- The operation of EF-Tu is thus similar to that of initiation
tion are much faster. The initial binding of the ternary factor 2 which binds charged initiator tRNA to the small
complex to the ribosome is readily reversed, but the inter- ribosomal subunit.
action is stabilized by the subsequent codon recognition Following dissociation from the ribosome the EF-Tu–
which induces the GTPase conformation of EF-Tu lead- GDP complex interacts with another elongation factor,
ing immediately to the hydrolysis of the GTP component EF-Ts, with formation of an EF-Tu–EF-Ts heterodimer
of the ternary complex to GDP. Hydrolysis of the GTP and release of GDP. Reaction of the heterodimer with
moiety causes a further change in the conformation of EF- GTP regenerates the EF-Tu–GTP complex required for
Tu from the GTP-binding to the GDP-binding form. This binding aminoacyl-tRNA. The sequence of events is simi-
conformational change leads to the release of aminoacyl- lar in eukaryotes with eEF-1A (Mr 50,000) corresponding
tRNA, allowing its CCA end to align with the peptidyl to EF-Tu and eEF-1B (Mr 30,000) to EF-Ts.
transferase center of the ribosome and the instantaneous Selection of the specific aminoacyl-tRNA to be bound
at the ribosomal A site is by base-pairing between the
relevant mRNA codon and the tRNA anticodon. Because
TABLE IV Properties of Prokaryotic Elongation and Termi-
nation Factors from E. coli this interaction involves only a triplet of bases and hence
a maximum of nine hydrogen bonds (see Fig. 1B), it is
M r (kDa) Properties and function
intrinsically unstable at physiological temperatures and
is probably stabilized by components of the ribosome to
Elongation factors
allow sufficient time for peptide bond synthesis to occur.
EF-T u 43 N-terminal acetyl-serine; heat labile; binds
aminaocyl-tRNA to the ribosomal A site Also, the codon–anticodon pairing must be monitored for
fidelity in order to minimize errors in translation. In E. coli
EF-T s 30 Heat stable; regeneration of EFT u –GTP
there is genetic and biochemical evidence that one of the
EF-G 77 GTP-dependent translocation of peptidyl-tRNA
and its mRNA codon from the A site to the proteins of the small ribosomal subunit, S12, is involved in
P site of the ribosome ensuring the fidelity of normal translation and in causing
Termination (release) factors the mistranslation which occurs in the presence of the
RF1 36 Requires UAA or UAG codons for hydrolysis antibiotic streptomycin due to incorrect codon-anticodon
of peptidyl-tRNA
interactions.
RF2 38 Requires UAA or UGA codons for hydrolysis
of peptidyl-tRNA
b. Translocation. Translocation involves the move-
RF3 46 Enhances RF1 and RF2 activity

ment of the ribosome along the mRNA in the 5 → 3