Isolation Of High S-adenosylmethionine Producing Strain By Complex Mutation And Optimization Of Culture Conditions

S-adenosylmethionine (SAM) is an important metabolite, and as a methyl group donor, participates in many reactions. It is utilized in three kinds of key metabolic pathways including transmethylation, polyamine synthesis and transsulfuration. Although SAM has been available as a prescription medication to improve many diseases such as depression, osteoarthritis, fibromyalgia, and liver disorders about two decades ago, it is expensive and out of reach for most people at this time. The reason is that there is no a more efficient way to produce it, also industrial-scale production of SAM is very limited.To obtain SAM-enriched strains, following works were done. In the first step, 572 strains were isolated from 69 different samples with isolation medium. A yeast strain named S42-12, the SAM content of which was 35.7 mg per gram biomass, was isolated by paper chromatography and HPLC.Secondly, Strain S42-12 was mutated by N-methy-N-nitro-N'- nitrosoguanidine (NTG), and the ethionine-resistant mutants were selected on the medium containing DL-ethionine. The SAM contents of the ethionine-resistant strains were measured to isolate SAM-accumulating mutants, and a strain named YA-89 was selected out, the SAM content (66.8 mg/g dry cell weight) was 1.87 times as much as that of S42-12. Cells of strain YA-89 were treated with NTG, and nystatin-resistant mutants were selected on plates contained 10μg/mL nystatin, a mutant named YN-34 was obtained. The SAM content of YN-34 is 71.6 mg/g cell weight. YN-34 was treated with UV irradiation, followed by resistance selection with 50μg/mL nystatin. A mutant YQ-5 was selected as a SAM-enriched mutant and the SAM content of YQ-5 (112.1 mg/g cell weight) was 214.09% higher than that of original stain.Finally, the optimal fermentation conditions and cultural media for the production of SAM was studied. The optimal medium composition contains 0.1% KH2PO4, 0.1% K2HPO4?3H2O, 0.03% MgSO4?7H2O, 0.01% CaCl2, 0.01% FeSO4?7H2O, 0.01% CoCl2, 0.01% CuSO4?5H2O, 0.03% H3BO3, 2% yeast extract, 8% sucrose, 1.5% peptone, 0.75% L-methionine. The SAM content of YQ-5 achieved 1740.0 mg/L, and the biomass was 14.6 g/L.