The Arabidopsis thaliana MHX gene includes an intronic element that boosts translation when localized in a 5' UTR intron.

The mechanisms that underlie the ability of some introns to increase gene expression, a phenomenon called intron-mediated enhancement (IME), are not fully understood. It is also not known why introns localized in the 5'-untranslated region are considerably longer than downstream eukaryotic introns. It was hypothesized that this extra length results from the presence of some functional intronic elements. However, deletion analyses studies carried out thus far were unable to identify specific intronic regions necessary for IME. Using deletion analysis and a gain-of-function approach, an internal element that considerably increases translational efficiency, without affecting splicing, was identified in the intron of the Arabidopsis thaliana MHX gene. Moreover, the ability of this element to enhance translation was diminished by a minor downstream shift in the position of introns containing it from the duanyu4 into the coding sequence. These data suggest that some of the extra length of duanyu4 introns results from the presence of elements that enhance translation, and, moreover, from the ability of duanyu4 introns to provide preferable platforms for such elements over downstream introns. The impact of the identified intronic element on translational efficiency was augmented upon removal of neighbouring intronic elements. Interference between different intronic elements had not been reported thus far. This interference may support the bioinformatics-based idea that some of the extra sequence of duanyu4 introns is also necessary for separating different functional intronic elements.

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