As a result of harboring obligatory bacterial endosymbionts, the xD strain of Amoeba proteus no longer produces its own S-adenosylmethionine synthetase (SAMS). When symbiont-free D amoebae are infected with symbionts (X-bacteria), the amount of amoeba SAMS decreases to a negligible level within four weeks, but about 47% of the SAMS activity, which apparently comes from another source, is still detected. Complete nucleotide sequences of sams genes of D and xD amoebae are presented and show that there are no differences between the two. Long-established xD amoebae contain an intact sams gene and thus the loss of xD amoeba's SAMS is not due to the loss of the gene itself. The open reading frame of the amoeba's sams gene has 1,281 nucleotides, encoding SAMS of 426 amino acids with a mass of 48 kDa and pI of 6.5. The amino acid sequence of amoeba SAMS is longer than the SAMS of other organisms by having an extra internal stretch of 28 amino acids. The 5′-flanking region of amoeba sams contains consensus-binding sites for several transcription factors that are related to the regulation of sams genes in E. coli and yeast. The complete nucleotide sequence of the symbiont's sams gene is also presented. The open reading frame of X-bacteria sams is 1,146 nucleotides long, encoding SAMS of 381 amino acids with a mass of 41 kDa and pI of 6.0. The X-bacteria SAMS has 45% sequence identity with that of A. proteus.