| Summary: | Heat shock protein 90 (Hsp90) is a molecular chaperone highly conserved across the species from prokaryotes to eukaryotes. Hsp90 is essential for cell viability under all growth conditions and is proposed to act as a hub of the signaling network and protein homeostasis of the eukaryotic cells. By interacting with various client proteins, Hsp90 is involved in diverse physiological processes such as signal transduction, cell mobility, heat shock response and osmotic stress response. In this research, we cloned the <em>dshsp90</em> gene encoding a polypeptide composed of 696 amino acids from the halotolerant unicellular green algae<em> Dunaliella salina</em>. Sequence alignment indicated that DsHsp90 belonged to the cytosolic Hsp90A family. Further biophysical and biochemical studies of the recombinant protein revealed that DsHsp90 possessed ATPase activity and existed as a dimer with similar percentages of secondary structures to those well-studied Hsp90As. Analysis of the nucleotide sequence of the cloned genomic DNA fragment indicated that <em>dshsp90</em> contained 21 exons interrupted by 20 introns, which is much more complicated than the other plant <em>hsp90</em> genes. The promoter region of <em>dshsp90</em> contained putative <em>cis</em>-acting stress responsive elements and binding sites of transcriptional factors that respond to heat shock and salt stress. Further experimental research confirmed that <em>dshsp90</em> was upregulated quickly by heat and salt shock in the <em>D. salina</em> cells. These findings suggested that <em>dshsp90</em> might serve as a component of the early response system of the <em>D. salina</em> cells against environmental stresses.
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