Intron-mediated enhancement of transgene expression in maize is a nuclear, gene-dependent process

In monocots, transgene expression can be stimulated by over two magnitudes by including an intron in the 5' untranslated region (UTR). The underlying mechanism is presently unknown. Inclusion of the salT intron into the 5' UTR of cat and bar genes stimulated expression of the first gene only, indicating that intron-mediated enhancement of expression (IME) is gene-dependent. Stimulation was associated with increased cat RNA levels, which did not result from a reduced cytoplasmic turnover and were not associated with increased translation. This implies that IME acts in the nucleus. Importantly, the cytoplasmic accumulation of spliced cat transcripts, even with IME, is less than that encoded by the intronless bar gene. As the cat and bar genes were flanked by identical regulatory signals, and the transcripts had a similar cytoplasmic stability it may mean that IME rescues rather than stimulates gene expression.