INTRONS ARE IMPORTANT PLAYERS OF TUBULIN GENE EXPRESSION: WHAT'S THE ROLES?

The control of gene expression in eukariotes is the result of a complex network of pathways acting at multiple steps from transcription to translation. This control is exerted by a strict cooperation between proteins and nucleic acids. Among the most recent actors in this scenery are introns, whose regulatory role have emerged in higher eukariotes. IME (Intron mediated enhancement of gene expression) is the best known regulatory effect exerted by introns within a transcription unit. In rice plants, we have shown that introns of tubulin genes can sustain IME and, in addition, can also influence the site of gene expression, determining tissue-specificity: this should be considered when testing new promoters for transgene expression. Introns may contribute to the evolution of those gene families where coding sequences are subjected to strong functional constraints. In fact, intron sequences, that are left to evolve more freely, can contribute to new pattern of expression. Several examples of enhancing introns have been reported in plants, but their mode of action is still elusive. A series of recombinant plasmids, based on the regulatory sequence (promoter-leader-intron) of rice tubulin genes, have been used to get new insights on intron-mediated control of gene expression, through transient expression assays and transgenic plant production. As a result, we show that introns act post-transcriptionally, that splicing is fundamental for intron function and that different and specific determinants act in monocot and dicot species. Interestingly, low reporter gene expression from a rice promoter in Arabidopsis or tobacco cells, is not enhanced by the corresponding intron but can be rescued by the addition of a dicot intron.