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    Cloning and expression of the Vitreoscilla hemoglobin gene in Enterobacter aerogenes
    (Pleiades Publishing Inc, 2004) Erenler, SO; Gencer, S; Geckil, H; Stark, BC; Webster, DA
    The hemoglobins found in unicellular organisms show a great deal of chemical reactivity, protecting cells against oxidative stress, and hence have been implicated in a wider variety of potential functions than those traditionally associated with animal and plant hemoglobins. There are well-documented studies showing that bacteria expressing Vitreoscilla hemoglobin (VHb), the first prokaryotic hemoglobin characterized, have better growth and oxygen uptake rates than their VHb counterparts. Here, the expression of VHb, its effect on the growth and antioxidant enzyme status of cells under different culture conditions was studied by cloning the complete regulatory and coding sequences (vgb) for VHb in Enterobacter aerogenes. Contrary to what has been reported for Escherichia coli, the expression of vgb in E.aerogenes decreased several fold under 10% of atmospheric oxygen (approximate to2% oxygen) and its growth was not greatly improved by the presence of VHb. Measured either as viable cells or total cell mass, untransformed E. aerogenes grew better than the recombinant strains. At the late exponential phase, however, the vgb-bearing strain was determined to have a higher cell number and total cell mass than the strain bearing only the plasmid vector with no vgb insert. The VHb expressing strain also had an oxygen uptake rate several fold higher than its counterparts. Given that oxidative stress may occur upon elevated oxygen exposure and be balanced by the action of antioxidative compounds, the level of antioxidative response of E. aerogenes expressing VHb was also studied. The VHb expressing strain had substantially (1.5-2.6-fold) higher catalase activity than strains not expressing VHb. Both VHb+ and VHb- strains, however, showed similar levels of superoxide dismutase activity. The activity of both enzymes was also growth phase dependent. Stationary phase cells of all strains showed 2-5-fold higher activity for these enzymes than cells at the exponential phase.
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    Enhanced production of acetoin and butanediol in recombinant Enterobacter aerogenes carrying Vitreoscilla hemoglobin gene
    (Springer, 2004) Geckil, H; Barak, Z; Chipman, DM; Erenler, SO; Webster, DA; Stark, BC
    Microbial production of butanediol and acetoin has received increasing interest because of their diverse potential practical uses. Although both products are fermentative in nature, their optimal production requires a low level of oxygen. In this study, the use of a recombinant oxygen uptake system on production of these metabolites was investigated. Enterobacter aerogenes was transformed with a pUC8-based plasmid carrying the gene (vgb) encoding Vitreoscilla (bacterial) hemoglobin (VHb). The presence of vgb and production of VHb by this strain resulted in an increase in viability from 72 to 96 h in culture, but no overall increase in cell mass. Accumulation of the fermentation products acetoin and butanediol were enhanced (up to 83%) by the presence of vgb/VHb. This vgb/VHb related effect appears to be due to an increase of flux through the acetoin/butanediol pathway, but not at the expense of acid production.
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    Genetic engineering of Enterobacter aerogenes with the Vitreoscilla hemoglobin gene
    (Elsevier, 2003) Geckil, H; Gencer, S; Kahraman, H; Erenler, SO
    Hemoglobins in unicellular organisms, like the one here in the bacterium Vitreoscilla, have greater chemical reactivity than their homologues in multicellular organisms. They can catalyze redox reactions and may protect cells against oxidative stress. The ability of Vitreoscilla hemoglobin to complement deficiencies of terminal cytochrome oxidases in Escherichia coli also suggests that this hemoglobin can receive electrons during respiration. In this study, a recombinant strain of Enterobacter aerogenes engineered to produce the Vitreoscilla Hb was investigated with regard to its susceptibility to oxidative stress. The culture response to oxidative stress produced by exogenously applied hydrogen peroxide was characterized in terms of cell growth, survival and the activities of two key antioxidant enzymes (catalase and superoxide dismutase). The influence of the physiological state of the cells and different media upon these culture dynamics was determined. Results showed that the hemoglobin-expressing strain is quite distinct in terms of growth/survival properties and activity of antioxidant enzymes from that of non-hemoglobin counterparts. (C) 2003 Editions scientitiques et medicales Elsevier SAS. All rights reserved.