HOW DO CELLS MINIMIZE THE COST OF TRANSLATION?

Protein translation is a costly process as it extensively consumes production resources, raw material consumables and energy. Although translation is well-studied, we still do not fully understand how cells minimize the costs of translation while maintaining the desired expression level. Towards that aim, we utilized an integrated approach of synthetic biology, lab-evolution and deep-sequencing. Recently, a synthetic library with ~14K variants was created and used to study the effects of 5` region-elements over the expression level of a GFP gene in Escherichia coli. We evolved this library and follow the frequency of each variant at regular time-intervals. Since all variants are competing against each other, we can compute the relative fitness of each of them. A comparison between fitness and GFP expression level revealed a negative correlation, but, interestingly only above a certain threshold. We then defined "fitness residual", which is the difference between its measured fitness to the expected fitness given its GFP expression level. By analyzing fitness residuals we found that ribosome attenuation at the early elongation phase minimizes cost of translation. Interestingly, this attenuation is reached by three independent mechanisms: First is a tight mRNA secondary structure. Second, is a high affinity to the ribosome anti- Shine Dalgarno motif. Third, a ribosomal-flow model suggests that positive residual variants show deeper ribosomal bottle-necks than negative variants. In addition, we found that incorporation of amino acids with a low energetic-cost is associated with a positive fitness residual. In conclusion, our study identifies molecular mechanisms that allow minimizing production costs at a desired expression level and deciphers the economy of translation.