S-腺苷甲硫胺酸合成酶(S-adenosylmethionine synthetase, SAMS)會將甲硫胺酸和ATP催化生成細胞中主要的甲基提供者S-腺苷甲硫胺酸(S-adenosylmethionine, SAM)。高鹽甲烷太古生物Methanohalophilus portucalensis FDF1T 可以從胞外攝取甜菜鹼或是以SAM作為甲基提供者將Glycine三次甲基化自體生合成甜菜鹼，在胞內累積作為相容質以平衡胞內外滲透壓。經由南方墨點法、M. portucalensis FDF1T metagenomic sequence資料與比較基因體分析發現M. portucalensis FDF1T有兩套不同型的sams基因，Mpsams1和Mpsams2，其中Mpsams1位於glycine betaine生合成基因組的上游。由序列分析得知催化活性相關的胺基酸和功能區塊與其他物種的SAMS有高度的相似性，推測兩套SAMS皆有催化生成SAM的活性。進一步序列比對與結構預測分析發現MpSAMS2在和phosphate binding，Mg2+ binding、K+ binding的主要保守性胺基酸與Mpsams1不同，且這些取代胺基酸在太古生物中具高度保守性。進一步演化歸群分析將MpSAMS1與細菌和真核SAMS歸群在一起，MpSAMS2則和其他太古生物的SAMS歸群在一起，顯示MpSAMS1可能藉由水平傳播的方式獲自其他物種，而MpSAMS2則是由太古生物的祖先垂直傳播來的。 S-adenosylmethionine synthetase (SAMS, EC 188.8.131.52) can catalyse the formation of S-adenosylmethionine which is the major methyl donor from methionine and ATP. Halophilic methanoarchaeon Methanohalophilus portucalensis FDF1T can de novo synthesis glycine betaine by stepwise methylation of glycine by using SAM as the methyl donor. Metagenomic and comparative genomics analysis of genus Methanohalophilus revealed there are two sams genes. One of them located up stream of glycine betaine synthesizing gene cluster, named Mpsams1. The complete gene of Mpsams1 was cloned by Southern hybridization. The other sams were amplified from FDF1T chromosome DNA by PCR according to FDF1T metagenomic pyrosequencing databse, named Mpsams2. Amino acid sequence analysis and homology modeling structure of these two MpSAMS exhibit conserved methionine binding site, ATP binding site, Mg2+ and K+ binding site with other species, suggesting both possess catalytic activities of SAM formation. Sequence alignment and putative structure revealed MpSAMS2 displayed the amino acid substitution for the phosphate binding，Mg2+ binding、K+ binding sites which were important for SAMS activity in E. coli. However, these substitutions are conserved among archaeal SAMS. Phylogenetic analysis revealed MpSAMS1 was clustered with SAMS from eukaryote and bacteria, whereas MpSAMS2 was clustered with SAMS from archaea. We hypothesized that MpSAMS1 were horizontally transfered from bacteria, whereas MpSAMS2 were vertically obtained from archaeal ancestor.