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Encyclopedia of Physical Science and Technology EN013D-617 July 27, 2001 11:42
Protein Synthesis 233
X. TRANSLATIONAL ACCURACY mutations in tRNA genes, typically in the anticodon, that
allow them to recognize a misplaced stop codon or un-
A. Types of Potential Errors intended frameshift. Suppressor tRNAs may be amino-
acylated according to the amino acid specificity of their
Given the complexity of protein synthesis, there are many
parent sequence, so therefore they insert this amino acid
steps at which mistakes can occur, and it is remarkable
into the polypeptide at the location of the mRNA aberra-
that the error rate is as low as it is. The two principal
tion. Alternatively, they may be misacylated because of the
categories of translational mistakes are missense errors,
change in the anticodon sequence. As long as the inserted
in which one amino acid is substituted for another or a
amino acid is not detrimental to the synthesized protein, a
stop codon is not recognized, and errors of processivity,
suppressor tRNA rescues the gene mutation. For example
in which frameshifting or premature termination occur.
the E. coli su6 suppressor tRNA inserts leucine into the
Missense errors can take place because of misacylation
growing polypeptide chain at the position of a premature
of a tRNA by an AARS, resulting in insertion of an incor-
UAA stop codon.
rect amino acid despite a correct codon–anticodon match.
Missense suppressors simply substitute one amino acid
This potential problem is counteracted by active editing
for another to correct a mutation in the mRNA. Trans-
by AARSs (see earlier), which is critical for maintaining
fer RNAs that suppress +1 frameshifts typically contain
translational accuracy. Errors can also occur at the level of
an extra base in the anticodon and read a four-nucleotide
the codon–anticodon interaction, such that a noncognate
codon, thereby restoring the correct translational frame.
AA–tRNA is selected at the ribosome, again resulting in
Suppressor tRNAs are of particular interest as more ex-
amino acid misincorporation. The relative abundance of
amples of the genetic basis for specific diseases are found.
cognate vs. noncognate AA–tRNA affects the accuracy
In many cases, missense or nonsense mutations in genes
of amino acid incorporation. In particular when the next
for essential proteins correlate with diseases such as can-
amino acid specified by the mRNA is unavailable, the
cer, cystic fibrosis, and amyotropic lateral sclerosis (ALS).
frequency of missense errors increases. A special case
In fact, mutations in the tumor suppressor gene p53 are the
of incorrect AA–tRNA selection is that of stop codon
single greatest cause of human cancers. One particular p53
readthrough, in which an amino acid is inserted where
hotspot is at codon 213, where nonsense mutations have
the protein should actually be terminated. This type of er-
been identified in human colorectal, gastric, ovarian, and
ror results in a carboxy-terminal extension (added to the
breast cancers. Suppressor tRNAs designed to selectively
nascent protein) that continues until the ribosome reaches
rescue these mutations by reading through the premature
the next stop signal.
termination codon could be effective therapeutic agents
Frameshift mutations occur because of a nucleotide in-
in a gene therapy approach. Several challenges remain,
sertion or deletion in a protein’s gene. Errors in reading
however—the suppressor tRNA must be introduced into
frame also occur when the ribosome “slips” along the
the affected cells, must be transcribed at high enough lev-
mRNA in such a way that the sequence is not read in
els to be effective, must also be aminoacylated at high
triplets corresponding to the codons of the message, but
levels, and must not excessively read through stop codons
the ribosome moves two or four nucleotides instead. These
that specify the normal termination of translation for
“slips” are −1or +1 frameshifts, respectively. Fortunately
other genes. These are active areas of research in several
the ribosome usually encounters a stop codon shortly after
laboratories.
the frameshift; this minimizes the effect of the mutation.
(Although frameshifting can be the unintended result of
translational inaccuracy, “programmed” frameshifts also C. Selenocysteine Insertion at UGA Codons
take place in special situations. These produce alternate
The trace element selenium is present in a number
polypeptides from a single mRNA.) Premature termina-
of proteins in the form of cotranslationally inserted
tion of a protein can occur when a nucleotide substitution
selenocysteine (Sec). This insertion occurs when a unique
produces a stop codon in the middle of the gene sequence. Sec
selenocysteinyl–tRNA recognizes an internal UGA
The peptidyl–tRNA may also dissociate prematurely from
codon. Selenocysteine has therefore been called the “21st
the ribosome before the stop codon is reached.
amino acid” and its insertion at a nucleotide triplet that is
normally a termination signal represents an expansion of
the genetic code.
B. Suppressor tRNAs
Insertion of selenocysteine requires several adaptations
For several of these error scenarios, faithful translation of of the translational machinery. First, tRNA Sec is amino-
the genetic message is accomplished through the action of acylated with serine by seryl–tRNA synthetase. The ser-
suppressor tRNAs. These are the result of advantageous ine attached to this misacylated species is then converted