Some key carotenogenic genes (crts) in Dunaliella bardawil are regulated in response to salt stress partly due to salt-inducible cisacting elements in their promoters. Thus, we isolated and compared the z-carotene desaturase (Dbzds) promoter with other crts promoters including phytoene synthase (Dbpsy), phytoene desaturase (Dbpds), and lycopene b-cyclase1 (DblycB1) to identify salt-inducible element(s) in the Dbzds promoter. In silico analysis of the Dbzds promoter found several potential cis-acting elements, such as abscisic acid response element-like sequence, myelocytomatosis oncogene1 recognition motif, AGC box, anaerobic motif2, and activation sequence factor1 binding site. Remarkably, instead of salt-inducible elements, we found a unique regulatory sequence architecture in the Dbzds promoter: a hypoosmolarity-responsive element (HRE) candidate followed by a potential hypoosmolarity-inducible factor GBF5 binding site. Deletion experiments demonstrated that only HRE, but not the GBF5 binding site, is responsible for hypoosmotic expression of the fusion of Zeocin resistance gene (ble) to the enhanced green fluorescent protein (egfp) chimeric gene under salt stress. Dbzds transcripts were in accordance with those of ble-egfp driven by the wild-type Dbzds promoter. Consequently, Dbzds is hypoosmotically regulated by its promoter, and HRE is responsible for this hypoosmotic response. Finally, the hypoosmolarity mechanism of Dbzds was studied by comparing transcript profiles and regulatory elements of Dbzds with those of Dbpsy, Dbpds, DblycB1, and DblycB2, revealing that different induction characteristics of crts may correlate with regulatory sequence architecture.Carotenoids are a structurally diverse class of isoprenoids synthesized by all photosynthetic organisms and many nonphotosynthetic organisms, such as certain species of bacteria, fungi, and archaea (Goodwin, 1980). They possess many advantageous properties for the human body on account of their vitamin A activity as essential nutrients (Farré et al., 2010), prevention and treatment functions against several kinds of diseases as health care products (Michaud et al., 2000;Landrum and Bone, 2001;Shaish et al., 2006), as well as industrial agents as colorants, forages, and cosmetics (SchmidtDannert, 2000). Therefore, the investigation of biosynthetic mechanisms and commercial exploitation of carotenoids have gained increasing attraction in many laboratories and companies. Recently, at least 700 carotenoids have been characterized from natural carotenoid biosynthetic pathways (Feltl et al., 2005).Some carotenogenic microorganisms have been commercially employed to produce important carotenoids (Johnson et al., 1995;Raja et al., 2007). Among these microorganisms, the Dunaliella genus, especially Dunaliella salina and Dunaliella bardawil, has been researched extensively and exploited as a natural source of carotenoids due to its striking ability to accumulate carotenoids under certain circumstances (Amotz et al., 1982), including high light intensity, high sa...
