Glutathione S-transferases(GSTs)are a superfamily of multifunctional proteins present in all organisms.GSTs catalyze the conjugation of reduced glutathioe with xenobiotics,to form more soluble,nontoxic compounds,ready to be excreted or compartmentalized.So GSTs are primarily involved in detoxification and antioxidant defense.So far,compared to higher plants and mammals,there is limited information on genes related to detoxification or antioxidant defense from Porphyra yezoensis Ueda.In order to investigate the contribution of GST to detoxification in thalli ofP.yezoensis, molecular cloning and expression analysis of a glutathione S-transferase gene(designated as PyGST)from P.yezoensis were performed.The genomic DNA and cDNA sequences of PyGST were obtained with computer assisted cloning,and characterized using multiple bioinformatic programs.The relative mRNA expression levels of PyGST under lead stress were investigated in thalli of P.yezoensis using realtime quantitative PCR.The PyGST cDNA contained a 624 nt of continuous complete coding region,encoding a polypeptide of 207 amino acids with a calculated molecular mass of 22.6 ku.The alignment of the genomic DNA with the cDNA showed that PyGST contained a 248 bp of intron,whose ends were defined by the 5′-GT and 3′-AG rule.PyGST contained conserved Nand Cdomains of GST family.The amino acids defining the binding sites of glutathione and xenobiotic substrates were also conserved.Sequence comparison of PyGST revealed 28.0%-39.6% and 23.0%-28.5% identity with most algal GSTs and animal Sigma class GSTs,respectively.PyGST was closely clustered with most algal GSTs in the phylogenetic tree.Phylogenetic tree also showed that most algal GSTs including PyGST were distinct from previously described GST classes,but were most closely related to the Sigma class.These features indicated that the PyGST belongs to Sigma-like GST.Lead stress significantly induced the expression of PyGST.The expression levels of PyGST induced by low concentrations of lead were higher than those by high concentrations of lead.Upon lead exposure(10 mg/L)for 12 h,the higher expression levels of PyGST were observed at 6 h and 12 h.These findings indicated that PyGST plays a role in detoxification of lead in thalli of P.yezoensis.