Deletion of iscR stimulates recombinant clostridial Fe–Fe hydrogenase activity and H2-accumulation in Escherichia coli BL21(DE3)

Proteins that catalyze H2-pathways often contain iron–sulfur (Fe–S) clusters and are sensitive to O2. We tested whether deletion of the gene encoding the transcriptional negative regulator, IscR, could enhance the ability of Escherichia coli BL21 to synthesize active recombinant H2-pathway components and stimulate ferredoxin-dependent H2-accumulation in the presence or absence of oxygen. Under anoxic conditions, deletion of iscR stimulated recombinant Fe–Fe hydrogenase activity threefold, whilst plasmid-based overexpression of the isc operon had no effect on hydrogenase activity. After cultivation with 21% (v/v) O2 in the headspace, no recombinant hydrogenase activity was observed in soluble extracts of wild-type BL21, although low levels of activity could be observed in the ΔiscR strain (700-fold lower than anoxic conditions, 180-fold greater than the limit of detection). Under closed batch conditions starting with 5% (v/v) O2, ΔiscR strains displayed fivefold greater levels of total hydrogenase activity and recombinant ferredoxin-dependent H2-accumulation relative to the control strain. In cultures starting with 10% (v/v) O2, H2-accumulation was stimulated 35-fold relative to the control. ΔiscR strains displayed enhanced synthesis and activity of integral H2-pathway components under all tested conditions and enhanced H2-accumulation under partially oxic conditions. Deletion of iscR is, therefore, a useful strategy to stimulate H2-production, particularly if the hydrogenase catalyzes the rate-limiting reaction.