cell news 2/2013
28
research news
repetitive sequences – an obstacle for the translation
paul saffert, zoya ignatova
each codon is translated with different speed
Translation is the last step in converting the genetic informati-
on into specifc physiological function. The major player in this
process is the ribosome, which, with the help of various factors
(including initiation, elongation and termination factors), trans-
lates the information stored in mRNA codon by codon into a
polypeptide. The degeneracy of the genetic code allows for more
than one possibility to encode for one amino acid: 61 sense co-
dons are used to encode for 20 amino acids. Synonymous co-
dons (i.e., codons coding the same amino acid) are not uniformly
used: some codons are more frequently used than others (1). To
each single codon a cognate transfer-tRNA (tRNA) pairs which is
loaded with an amino acid by the corresponding aminoacyl-tR-
NA-synthestase (2). The tRNA species largely differ in their con-
centrations (3-4). Since charged tRNAs arrive at the ribosome
solely by diffusion, the concentration of each tRNA species will
signifcantly infuence the rate of translation of each codon (5);
codons read by more abundant tRNAs will be translated faster as
opposed to codons paring to minor tRNAs. In unicellular orga-
nisms the relative abundance of codons correlates well with the
tRNA gene copy number (3-4). In multicellular organisms, howe-
ver, the tRNA concentration differ signifcantly between various
tissues (6), despite the uniform codon usage and common tRNA
gene set of each cell. Codon distribution along the mRNA de-
termines the speed of translation of each mRNA; clustering of
codons read by lowly-abundant tRNAs causes transient pauses
in the ribosome movement (7). This non-uniform distribution of
slow and fast translated codons determines the speed of transla-
tion initiation (8), guides co-translational folding of the nascent
protein (9-10), translocation or membrane insertion (11).
Translation is highly dynamic process and high demand of a cer-
tain codon or codon group may reduce the apparent concentra-
tion of the cognate charged tRNA, thus artifcially converting an
abundant codon in an apparent rare codon (5, 12). This depends
on the initial concentration of a tRNA species in a cell or tis-
sue. The effect is expected to be non-uniform for each tRNA
and might be restricted to only certain tissues of a multicellular
organism.
cag repeat diseases
Huntington’s disease (HD) is an inherited neurodegenerative dis-
order, which is associated with an expansion of a CAG stretch,
encoding for glutamine, within the N-terminal exon 1 of hun-
Figure 1. CAG repeats are prone to translational frameshift at any
position within the CAG stretch:
(a) Schematic of the reporter of -1 frameshifting, Htt51Q(-1)YFP.
(b) N2a, N2a/Htt65Q-CFP, and N2a/Htt103Q-CFP cells were transiently
transfected with the Htt51Q(-1)YFP reporter and visualized by fuores-
cence (YFP) and phase contrast (phase) microscopy after 24 hr (left panel).
Htt51Q and Htt51QYFP expression was monitored by immunostaining of
the N-terminal HA tag. Scale bar, 10 mm. The percentage of cells contai-
ning YFP-positive, frameshifted aggregates from the total amount of cells
transfected with Htt51Q(-1)YFP (i.e., HA-positive) was quantifed from the
microscopy images (right panel). The values are expressed as means of fve
independent experiments ± SEM. **p < 0.01.
(c) MALDI-TOF-MS analysis of N2a/Htt103Q-CFP cells expressing Htt51(-1)
YFP for 24 hr, coimmunoprecipitated with GFP-antibodies, subsequently
separated by 2D-gel electrophoresis. Figure adopted from (19).
