S-Adenosyl-L-methionine(SAM)is the most common methyl donor in a multitude of cellular methylation reactions. Numerous methyltransferases transfer the methyl group from SAM to their respective biological acceptors, forming S-adenosyl-L-homocysteine(SAH). SAH is hydrolyzed to adenosine and L-homocysteine by S-adenosyl-L-homocysteine hydrolase(SAHH, EC 3.3.1.1). SAHH is an essential enzyme in all living cells, and has been explored as a potential drug target for many bacteria and parasites. The inhibition of SAHH activity can result in the accumulation of SAH and reduce the S-adenosylmethionine(SAM):SAH ratio in cells. In that case, SAHH can act as a potent feedback inhibitor, blocking the SAM-dependent methylation that is required for the metabolism of a wide variety of biological compounds such as nucleic acids, proteins, phospholipids, and other small molecules. SAHH is found in animals, plants, fungi, and other microorganisms. Antarctic ice microalgae with special characteristics to withstand the extreme environment characterized by low temperatures and ice, high levels of dissolved oxygen, and strong seasonal changes in light intensity have been investigated in recent years. To further understand the intrinsic mechanisms by which the Antarctic sea ice alga sp. ICE-L responds to ecological and environmental factors, we cloned and analyzed its SAHH. A cDNA encoding S-adenosyl homocysteine hydrolase(designated as ICE-LSAHH)was cloned from this alga by RT-PCR and RACE-PCR methods. The ICE-LSAHH full-length cDNA sequence was 1844 bp, including a 5-terminal UTR of 344 bp with a poly(A)tail, and an open reading frame(ORF)of 1461 bp encoding a polypeptide of 487 amino acids. The predicted molecular weight(MW)of ICE-LSAHH was 53 kD with an estimated pI of 5.16. Using software, it was predicted that the ICE-LSAHH protein did not contain a signal peptide or a transmembranous region, suggesting that it is not a secretory protein. To further analyze the evolutionary relationship among SAHH enzymes, a molecular phylogenetic tree was constructed using ClustalX 2.0 and Mega 5.0 software. The reliability of branching was tested by bootstrap re-sampling(1 000 pseudo-replicates). In the phylogenetic tree, members of the SAHH protein formed clear subgroups of plants, animals, algae, and bacteria. The . Sequence analysis with the BLAST algorithm showed that the ICE-LSAHH gene shared 62%–85% sequence identity with other SAHHs. The highest sequence identity was 85% with the SAHH gene of . Amino acid sequence alignment between ICE-LSAHH and other SAHHs showed that the ICE-LSAHH protein exhibited high sequence homology with other SAHHs. The three-dimensional structure of ICE-LGPx was determined using SWISS-MODEL workspace and software. The 3D molecular model showed that the ICE-LSAHH subunit has three domains: a substrate-binding domain, an NAD-binding domain, and a C-terminal domain. In conclusion, Chlamydomonas sp. ICE-L.