Catalyzes the formation of S-adenosylmethionine from methionine and ATP. The reaction comprises two steps that are both catalyzed by the same enzyme: formation of S-adenosylmethionine (AdoMet) and triphosphate, and subsequent hydrolysis of the triphosphate. ATP + H2O + L-methionine = diphosphate + phosphate + S-adenosyl-L-methionine Binds 2 divalent ions per subunit. The metal ions interact primarily with the substrate (By similarity). Can utilize magnesium, manganese or cobalt (in vitro) (PubMed:9037140). Binds 1 potassium ion per subunit. The potassium ion interacts primarily with the substrate (By similarity). Potassium can be replaced by NH(4) (in vitro) (PubMed:9037140). Inhibited by products of SAMS reaction (SAM, Pi, PPi), substrate analogs (cycloleucine and ethionine), and alternative nucleotides (GTP, CTP and ADP). Strongly repressed by PPPi. Optimum pH is 7-8.3. Optimum temperature is 37-45 degrees Celsius. Amino-acid biosynthesis; S-adenosyl-L-methionine biosynthesis; S-adenosyl-L-methionine from L-methionine: step 1/1. Homotetramer. Mostly expressed in roots, and, to a lower extent, in hypocotyls and cotyledons. Strongly induced by elicitors from Phytophthora megasperma. Transiently activated by salt stress,. Belongs to the AdoMet synthase family.