Deletion of gene gnd encoding 6-phosphogluconate dehydrogenase promotes L-serine biosynthesis in a genetically engineered strain of Methylobacterium sp.  MB200. 
To identify potential target genes involved in L-serine biosynthesis in Methylobacterium sp.  MB200 and to evaluate the gnd genetically-engineered strains for L-serine production.  Five genes that are not associated with the central metabolic pathway but with L-serine biosynthesis were identified from Methylobacterium sp.  MB200 mutants.  Gene gnd, encoding 6-phosphogluconate dehydrogenase (PGDH), was selected for further evaluation.  The gnd deletion mutant showed a 600% increase in D-serine tolerance and an 80% decrease in PGDH activity compared to Methylobacterium sp.  MB200.  gnd over-expression did not affect D-serine tolerance, whereas it did increase enzyme-activity up to 136%.  Additionally, analysis revealed that in Methylobacterium sp.  MB200, L-serine inhibited PGDH activity.  The deletion of gnd did not affect growth, whereas it did enhance the biosynthesis of L-serine, resulting in a 225% increase in production of L-serine compared to the wild-type.  gnd, one of the five genes identified here that is associated with L-serine synthesis, can be developed as a potential candidate for metabolic engineering to promote L-serine synthesis in Methylobacterium sp.  MB200.