Intron-mediated enhancement is not limited to introns

Certain introns strongly increase mRNA accumulation by a poorly understood mechanism known as Intron-Mediated Enhancement (IME). Introns that boost expression by IME have no effect when located upstream of or more than ~1 Kb downstream from the start of transcription. The sequence TTNGATYTG, which is over-represented in promoter-proximal introns in Arabidopsis thaliana, can convert a non-stimulating intron into one that strongly increases mRNA accumulation. We tested the ability of an intron containing this motif to stimulate expression from different locations and found that it had the same positional requirements as naturally occurring IME introns. The motif also stimulated gene expression from within the 59-UTR and coding sequences of an intronless construct. Furthermore, the 59-UTR of another gene increased expression when inserted into an otherwise non-stimulating intron in coding sequences. These results demonstrate that splicing is not required for intron-mediated enhancement, and suggest that other sequences downstream of the transcription start site in addition to introns may stimulate expression by a similar mechanism.