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 (By similarity). May be involved in the synthesis of betain in response to abiotic stress such as high salinity (Ref.1). 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) (By similarity). Binds 1 potassium ion per subunit. The potassium ion interacts primarily with the substrate (By similarity). Amino-acid biosynthesis; S-adenosyl-L-methionine biosynthesis; S-adenosyl-L-methionine from L-methionine: step 1/1. Homotetramer. By salt stress. Follow a circadian regulation with higher levels in the light. Belongs to the AdoMet synthase family.