(63g) Inceasing Organic Acid Tolerance In Cyanobacteria

Due to escalating petroleum and commodity prices, there is increased interest in engineering photosynthetic microorganisms to directly produce fuel and chemicals from light and CO₂.  Of particular interest are the organic acids acrylate and 3-hydroxypropionate.  Acrylate is used in the manufacturing of plastics and coatings, while 3-hydroxypropionate can be catalytically converted to a variety of useful compounds, including acrylate.  One obstacle to engineering cyanobacteria to produce commodity chemicals, in particular organic acids, is toxicity.  The cyanobacterium Synechococcus sp. PCC 7002, a well studied organism with a relatively fast growth rate and amendable genome, is completely inhibited by 50µM acrylate.  In comparison E. coli is inhibited by 25mM.  Attempts to increase the tolerance of PCC 7002 to organic acids resulted in the identification of a mutant strain capable of growth in greater than 100mM acrylate, a 2000-fold increase.  In addition, this mutation resulted in 100 fold increases in tolerance to other commercially relevant organic acids including propionate and 3-hydroxypropionate.  This mutation was mapped to the acsA gene, which encodes an acetyl-CoA ligase.  A direct knock out of acsA resulted in a similar phenotype as the spontaneous mutant.  The utility of this strain is currently being demonstrated by engineering a 3-hydroxypropionate production pathway.