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The genetic code is degenerate; thus, protein evolution does not uniquely determine the coding sequence. One of the puzzles in evolutionary genetics is therefore to uncover evolutionary driving forces that result in specific codon choice. In many bacteria, the first 5-10 codons of protein-coding genes are often codons that are less frequently used in the rest of the genome, an effect that has been argued to arise from selection for slowed early elongation to reduce ribosome traffic jams. However, genome analysis across many species has demonstrated that the region shows reduced mRNA folding consistent with pressure for efficient translation initiation. This raises the possibility that unusual codon usage is a side effect of selection for reduced mRNA structure. Here we discriminate between these two competing hypotheses, and show that in bacteria selection favours codons that reduce mRNA folding around the translation start, regardless of whether these codons are frequent or rare. Experiments confirm that primarily mRNA structure, and not codon usage, at the beginning of genes determines the translation rate.
Background:
Environmental stress puts organisms at risk and requires specific stress-tailored responses to maximize
survival. Long-term exposure to stress necessitates a global reprogramming of the cellular activities at different
levels of gene expression.
Results:
Here, we use ribosome profiling and RNA sequencing to globally profile the adaptive response of
Arabidopsis thaliana
to prolonged heat stress. To adapt to long heat exposure, the expression of many genes is
modulated in a coordinated manner at a transcriptional and translational level. However, a significant group of
genes opposes this trend and shows mainly translational regulation. Different secondary structure elements are
likely candidates to play a role in regulating translation of those genes.
Conclusions:
Our data also uncover on how the subunit stoichiometry of multimeric protein complexes in plastids
is maintained upon heat exposure.