Analysis of novel hemoglobins during microaerobic growth of HMP-negative Escherichia coli

Limited oxygen availability is a prevalent problem in microbial biotechnology leading to reduced growth and productivity. Vitreoscilla hemoglobin (VHb) has been extensively studied showing positive and energetically favorable characteristics. Species-specific differences also suggest that VHb is not optimal for all applications. Therefore, mutant and fusion proteins were generated with improved characteristics. Hemoglobins of various microorganisms also provide phenotypes eliciting positive properties. Therefore, two new globin genes, encoding Novosphingobium aromaticivorans hemoglobin (NHb) and Bacillus halodurans flavohemoglobins (HmpBh), were isolated. Genes encoding two mutant flavohemoglobins of Pseudomonas aeruginosa PAO1 and a clinical isolate were also analyzed. The proteins were expressed to study the physiological effects of globin expression relative to controls, carrying either the parental plasmid or expressing VHb, during oxygen limited fed-batch cultivations of Escherichia coli BO6, lacking the endogenous flavohemoglobin. HmpBh-expressing cells reached 25 and 10% higher final optical densities relative to the plasmid carrying or VHb-expressing cells, respectively. HmpBh-expressing cells also showed 34% higher yield of biomass on glucose relative to control. Sequence comparison indicates that hemoglobin domains of Bacillus species are closely related to VHb sequence. Thus, suggesting that proteins having a globin domain similar to VHb could be important in biotechnology.