Increased H2 production in the cyanobacterium Synechocystis sp strain PCC 6803 by redirecting the electron supply via genetic engineering of the nitrate assimilation pathway

The unicellular cyanobacterium Synechocystis sp. strain PCC 6803 contains a single bidirectional NiFe-Hox-hydrogenase, which evolves hydrogen under certain environmental conditions. The nitrate assimilation pathway is a potential competing pathway that may reduce the electron flow to the hydrogenase and thereby limit hydrogen production. To improve H-2 production, the nitrate assimilation pathway was disrupted by genetic engineering to redirect the electron flow towards the Hox-hydrogenase. Mutant strains disrupted in either nitrate reductase (Delta narB) or nitrite reductase (Delta nirA) or both nitrate reductase and nitrite reductase (Delta narB:Delta nirA) were constructed and tested for their ability to produce hydrogen. H-2 production and Hox-hydrogenase activities in all the mutant strains were higher than those in wild-type. Highest H-2 production was observed in the Delta narB:Delta nirA strain. Small changes were observed for Hox-hydrogenase enzyme activities and only minor changes in transcript levels of hoxH and hoxY were not correlated with H-2 production. The results suggest that the high rate of H-2 production observed in the Delta narB:Delta nirA strain of the cyanobacterium Synechocystis sp. strain PCC 6803 is the result of redirecting the electron supply from the nitrate assimilation pathway, through genetic engineering, towards the Hox-hydrogenase. (C) 2011 Elsevier Inc. All rights reserved.