A Non-functional ?-Aminobutyric Acid Shunt Pathway in Cyanobacterium Synechocystis sp. PCC 6803 Enhances d-Aminolevulinic Acid Accumulation under Modified Nutrient Conditions

To redirect carbon flux from the gamma-aminobutyric acid (GABA) shunt to the delta-aminolevulinic acid (ALA) biosynthetic pathway, we disrupted the GABA shunt route of the model cyanobacterium Synechocystis sp. PCC 6803 by inactivating Gdc, the gene-encoding glutamate decarboxylase. The generated & UDelta;Gdc strain exhibited lower intracellular GABA and higher ALA levels than the wild-type (WT) one. The & UDelta;Gdc strain's ALA levels were similar to 2.8 times higher than those of the WT one when grown with levulinic acid (LA), a competitive inhibitor of porphobilinogen synthase. Abiotic stress conditions including salinity induced by 10 mM NaCl and cold at 4 ? increased the ALA levels in delta Gdc up to similar to 2.5 and 5 ng g(-1) cell DW, respectively. The highest ALA production in the delta Gdc cyanobacteria grown in BG11 medium was triggered by glucose induction, followed by glutamate supplementation with 60 mM of LA, thereby resulting in similar to 360 ng g(-1) cell DW of ALA, that is > 300-fold higher ALA accumulation than that observed in delta Gdc cyanobacteria grown in normal medium. Increased levels of the gdhA (involved in the interconversion of alpha-ketoglutarate to glutamate) and the hemA (a major regulatory target of the ALA biosynthetic pathway) transcripts occurred in delta Gdc cyanobacteria grown under modified growth conditions. Our study provides critical insight into the facilitation of ALA production in cyanobacteria.

Automated summary

To increase the production of delta-aminolevulinic acid (ALA) in cyanobacteria, the gamma-aminobutyric acid (GABA) shunt route was disrupted by inactivating the gene encoding glutamate decarboxylase. The resulting strain showed higher ALA levels when grown in the presence of levulinic acid (LA) and under salt and cold stress conditions. The highest ALA production was achieved by glucose induction and glutamate supplementation with LA. Increased levels of certain transcripts involved in ALA biosynthesis were observed in the disrupted strain under modified growth conditions.