In trangenic rice, α- and β-tubulin regulatory sequences control GUS amount and distribution through intron mediated enhancement and intron dependent spatial expression

The genomic upstream sequence of the rice tubulin gene OsTub6 has been cloned, sequenced and characterized. The 5'UTR sequence is interrupted by a 446 bp long leader intron. This feature is shared with two other rice beta-tubulin genes (OsTub4 and OsTub1) that, together with OsTub6, group in the same clade in the evolutionary phylogenetic tree of plant beta-tubulins. Similarly to OsTub4, the leader intron of OsTub6 is capable of sustaining intron mediated enhancement (IME) of gene expression, in transient expression assays. A general picture is drawn for three rice alpha-tubulin and two rice beta-tubulin genes in which the first intron of the coding sequence for the formers and the intron present in the 5'UTR for the latters, are important elements for controlling gene expression. We used OsTua2:GUS, OsTua3:GUS, OsTub4:GUS and OsTub6:GUS chimeric constructs to investigate the in vivo pattern of beta-glucuronidase (GUS) expression in transgenic rice plants. The influence of the regulatory introns on expression patterns was evaluated for two of them, OsTua2 and OsTub4. We have thus characterized distinct patterns of expression attributable to each tubulin isotype and we have shown that the presence of the regulatory intron can greatly influence both the amount and the actual site of expression. We propose the term Intron Dependent Spatial Expression (IDSE) to highlight this latter effect.