Improved methylation in E. coli via an efficient methyl supply system driven by betaine

Methylation reactions are involved in the biosynthesis of various natural molecules, in which S-adenosyl-L-methionine (SAM) acts as the principal biological methyl donor. The limited availability of SAM often affects the biosynthesis of methylated metabolites in cells, especially when heterologous SAM-mediated methyltransferases are employed. To solve this problem, a methyl supply system driven by betaine was developed in this study to enhance SAM availability in cells. A reconstructed methionine cycle was designed in E. coli using betaine as the methyl source by introducing betaine-homocysteine methyltransferase. Ferulic acid served as a model product was used to test the efficiency of methyl supply system. ATP is a co-factor for SAM biosynthesis and a pathway for ATP regeneration from adenosine was introduced to maintain the stability of the adenylate pool. After testing two different S-adenosyl-L-homocysteine (SAH) hydrolysis pathways, the optimized SAHase pathway was adopted for converting SAH back to homocysteine (Hcy). Thus, a methyl supply system was developed which increased SAM availability and therefore improved the titer and productivity of ferulic acid by 12.6-fold and 15.9-fold, respectively. The system was also applied successfully for other methyltransferase-catalyzed reactions. This work provides an efficient methyl supply system for enhanced production of methylated chemicals using betaine as the methyl source.