The salt-regulated element in the promoter of lycopene β-cyclase gene confers a salt regulatory pattern in carotenogenesis of Dunaliella bardawil

In the carotenoid biosynthesis, lycopene β-cyclase (LCYb) is a key regulatory enzyme involved in the conversion of lycopene into β-carotene. Under stress conditions, such as high salinity, high light and nutrient deprivation, large amounts of β-carotene can be accumulated in Dunaliella bardawil. To study on the molecular responses of salt stress in D. bardawil is of great significance to reveal the mechanisms of salt tolerance and engineer crop plants to be salt-tolerant. In this study, the full-length coding sequence of lcyb from D. bardawil (Dblcyb, GenBank: KX218392) was isolated by transcriptome sequencing. Then, the genomic sequence, promoter and terminator regions of Dblcyb were isolated by genome walking. The Dblcyb promoter (GenBank: KX218393) contained several typical transcription boxes, multiple light response elements and a salt-regulated element (SRE, GT1GMSCAM4). Dbpsy and Dblcyb responsible for β-carotene biosynthesis in D. bardawil was shown to be up-regulated under salt stress and their promoters contained the common SRE. By element deletion analysis and using Ble-EGFP as the reporter, the salt-inducible SRE was confirmed to confer salt-induced expression of Dblcyb promoter. It was indicated that the salt-regulated expression of Dblcyb may be attributed to the salt-responsive element (GT1GMSCAM4) and the GT-rich region in its genomic sequence.