The 90ku heat shock proteins(HSP90s)are a group of evolutionarily highly conservative molecular chaperones and essential for viability in eukaryotes.The basic functions of HSP90s are of assisting protein folding,protein degradation and protein trafficking,and they also play an important role in signal transduction networks,cell cycle control and morphological evolution.Although HSP90s are constitutively expressed in most organisms,their expression is upregulated in response to stresses such as cold,heat,salt stress,heavy metals,phytohormones,light and dark transitions. Porphyra yezoensis Ueda,one of intertidal macroalgae,periodically experiences immersion and emersion due to the tidal cycles.When P.yezoensis thalli are exposed to air,they experience a variety of abiotic stresses such as nutrient limitation,high light,ultraviolet ray,high and low temperature,desiccation and osmotic stress.In order to investigate the contribution of HSP90 to abiotic stresses tolerance in P.yezoensis thalli,molecular cloning and expression analysis of HSP90 gene(designated as PyHSP90)from P.yezoensis were performed. PyHSP90 gene was obtained with computer assisted cloning,and characterized using multiple bioinformatic programs.The relative mRNA expression levels of PyHSP90 gene under stresses were investigated in P.yezoensis thalli using real-time RT-PCR.The PyHSP90 cDNA contained a 2 274 nt of continuous complete open reading frame encoding a polypeptide of 757 amino acids with a calculated molecular mass of 86 ku.Analytic results of PSORT,TargetP and phylogenetic tree showed that PyHSP90 localized to cytoplasm.PyHSP90 contained conserved signature motifs and domains of HSP90 family.PyHSP90 shared high amino acid sequence identity with HSP90s from other organisms.Sequence comparison of PyHSP90 revealed 26.0%-39.4% and 61.4%-71.8% identity with prokaryotic and eukaryotic HSP90s respectively.In the phylogenetic tree red algae HSP90s,including PyHSP90,clustered with Cryptophyta HSP90s firstly,and then clustered with HSP90s of green algae and land plants,and at last all cytoplasmic HSP90s of algae,plants and animals were clustered together.Low and high temperature stresses significantly induced the expression of PyHSP90 gene in P.yezoensis thalli,and there was a clear dose-dependent expression pattern of PyHSP90 gene after different temperature exposure.The expression of PyHSP90 gene was up-regulated under low salinity stress,and downregulated under desiccation stress.The results indicated that PyHSP90 is essential to hold configuration of proteins under temperature and salinity stresses,and dispensable under desiccation stress.Investigation of PyHSP90 gene can contribute to its molecular mechanism of tolerance to abiotic stresses,and lay a foundation for further enhancing resilience of P.yezoensis through genetic improvement.