L-Serine plays a critical role as a building block for cell growth, and thus it is difficult to achieve the direct fermentation of L-serine from glucose. In this study, Corynebacterium glutamicum ATCC 13032 was engineered de novo by blocking and at- tenuating the conversion of L-serine to pyruvate and glycine, releasing the feedback inhibition by L-serine to 3-phosphoglycerate dehydrogenase （PGDH）, in combination with the co-expression of 3-phosphoglycerate kinase （PGK） and feedback-resistant PGDH （PGDHr）. The resulting strain, SER-8, exhibited a lower specific growth rate and significant differ- ences in L-serine levels from Phase I to Phase V as determined for fed-batch fermentation. The intracellular L-serine pool reached （14.22_＋1.41） ~trnol gcoM-1, which was higher than glycine pool, contrary to fermentation with the wild-type strain. Furthermore, metabolic flux analysis demonstrated that the over-expression of PGK directed the flux of the pentose phosphate pathway （PPP） towards the glycolysis pathway （EMP）, and the expression of PGDHr improved the L-serine biosynthesis pathway. In addition, the flux from L-serine to glycine dropped by 24%, indicating that the deletion of the activator GlyR re- sulted in down-regulation of serine hydroxymethyltransferase （SHMT） expression. Taken together, our findings imply that L-serine pool management is fundamental for sustaining the viability of C. glutamicum, and improvement of C1 units genera- tion by introducing the glycine cleavage system （GCV） to degrade the excessive glycine is a promising target for L-serine pro- duction in C. glutamicum.
LAI ShuJuanZHANG YunLIU ShuWenLIANG YongSHANG XiuLingCHAI XinWEN TingYi
