VHb (hemoglobin)

The VHb gene, vgb, exists as a single copy in Vitreoscilla and exhibits complete agreement with the primary sequence of VHb.^[6][17] The downstream region adjacent to vgb carries a gene in the opposite direction having close similarity with the uvrA gene of E. coli, indicating that vgb is not part of a multigene operon.^[18] Biosynthesis of VHb is regulated at the transcriptional level and is induced under hypoxia in its native host.^[19] vgb is expressed strongly in E. coli through its native promoter and a similar increase in its transcript level occurs under hypoxia; this suggests a close similarity in the transcriptional machinery of Vitreoscilla and E. coli.^[20] The promoter region of vgb is crowded with overlapping binding sites for several redox-sensitive transcriptional regulators, involving the fumarate and nitrate reduction (Fnr) system as primary regulator.^[15] The catabolite repression (Crp) system is an additional control^[21] along with the aerobic respiration control (Arc) system as a third oxygen-dependent controller.^[22] Another binding site for the oxidative stress response regulator (OxyR) is also present within the vgb promoter; all these transcriptional regulators appear to work in coordination with each other to control the biosynthesis of VHb in a redox dependent manner.^[15]

Since it was shown that VHb stimulated the growth of E. coli under hypoxic conditions, vgb was cloned into a variety of organisms, including various bacteria, yeast, fungi, and even higher plants and animals to test its effects on growth and production of products of potential commercial importance, the degradation of toxic compounds, the enhancement of nitrification in wastewater treatment, and other environmental applications.^[23]

Examples of increased productivity include increased yield of a variety of biochemicals including antibiotics, an insecticide, a surfactant, and potential plastic feedstocks. They also include enzymes^[24] (including one which might have anti-leukemic properties), and fuels (including ethanol,^[25] butanediol,^[26][27] and biodiesel^[28]). The toxic compounds studied have been aromatics including 2-chlorobenzoic acid and 2,4-dinitrotolene.^[29][30] In these cases, increases in degradation are thought to be due both to the effects of VHb enhancing respiration to provide cells with additional ATP for growth and production of degrading enzymes, and delivery of oxygen directly to the oxygenases required for early steps in the degradative pathways.

Other environmental investigations include those related to heavy metal remediation and provision of soil phosphate to plants.^[28] Expression of vgb in Nitrosomonas europaea, a bacterium involved of conversion of ammonia to nitrite in wastewater, enhanced, to some degree, its ability in this conversion.^[31] Furthermore, it was shown that the mechanism of haeme protein expression to enhance oxygen supply to the monooxygenase in nitrification under hypoxic conditions is similar to VHb function seen in other applications.^[32]

Amino acid residues in several sections of VHb in proximity to the haeme were altered using genetic engineering to change VHb's affinity for oxygen and to examine the effects on the biotechnological properties of some of the systems studied.^[23] Many of the mutations did not have large effects on the ligand binding properties of VHb, or provided at best a modest increase in cell growth compared with cells harboring wild type VHb.^[23][33] Two of the mutant VHb's, however, provided substantial increases in growth and aromatic compound degradation compared to wild type VHb in Pseudomonas and Burkholderia bacteria transformed to contain vgb.^[34]