Introduction of an AU-rich Element into the 5' UTR of mRNAs Enhances Protein Expression in Escherichia coli by S1 Protein and Hfq Protein

AU-rich elements in 5' untranslated region (UTR) are known to increase translation efficiency by recruiting S1 protein that facilitates the assembly of ribosomes. However, AU-rich elements also serve a binding site for Hfq protein, RNase E, etc. To investigate their roles in translation, mRNAs containing either an AU-rich element, originated from sodB 5'-UTR or a non-AU-rich element were constructed. The non-AU-rich elements were designed to retain the thermodynamics of the AUrich element-containing mRNAs to reduce structural effect on translation. The AU-rich element increased mRNA translation and knock-down of S1 protein decreased the translation of AU-rich element-containing mRNAs, confirming the essential role of S1 protein in translation. When their mRNA levels were measured in hfq-deleted cells, those containing a non-AU-rich element and a high AU-content N-terminal coding sequence decreased, representing an auxiliary role of Hfq in translation, specifically in mRNA protection. Interestingly, despite of decreased mRNA level in hfq-deleted cells, protein production was increased, implying the involvement of unknown factors in translation. Consequently, these results suggest that actively translating ribosomes recruited by S1 protein at an AU-rich element stabilize mRNAs from degradation. In the absence of S1 protein, Hfq protein protects mRNAs from degradation. Moreover, AU-rich elements can be used for improved protein production.