Abstract

S-adenosylmethionine (SAM) is a universal metabolite formed from methionine and ATP. SAM has a highly reactive sulfonium group and is used as methyl donor in a large number of methylations. The metK gene, coding for the enzyme which synthesizes SAM, SAM synthetase, is an essential gene in E. coli. However the study of the importance of SAM has been hindered by the inability of the cells to incorporate this compound directly from the medium. A plasmid carrying the SAM transporter from the intracellular parasitic bacterium Rickettsia prowazekii was used to construct an E. coli strain lacking a functional copy of metK. The resulting strain requires a supply of exogenous SAM for growth, demonstrating that SAM is the only essential product of SAM synthetase. The use of the transporter in strain MetK84 confirmed that the filamenting phenotype observed in the strain is explained by its low intracellular concentration of SAM. Studies on the relation of SAM concentration and production of biomass in a strain carrying the SAM transporter showed a linear relation at low SAM concentrations, but suggested that SAM is wasted at high concentrations.