Effect of the oxygen transfer rate on oxygen-limited production of plasmid DNA by Escherichia coli

The influence of oxygen limitation on pDNA yields in cultures of Escherichia coll. was studied. Cultures at maximum oxygen transfer rate (OTRmax) values of 10, 14, 30 and 45 mmol L-1 h(-1) and an aerobic culture at an OTRmax of 110 mmol L-1 h(-1) were performed in microtiter plates (MTPs). Dissolved oxygen tension (DOT), pH, biomass and NADH fluorescence were monitored online. An E. coll. strain that constitutively expresses Vitreoscilla hemoglobin (VHb) was used and compared with its parent strain. pDNA yields were inversely proportional to the OTRmax for both strains and increased more than two-fold in cultures at the lowest OTRmax compared with that in aerobic cultures. Expression of VHb increased the specific growth rate at OTRmax values of 10, 14 and 30 mmol L-1 h(-1) compared with that of the parent strain. NADH-specific fluorescence decreased in cultures of the engineered strain. The pDNA supercoiled fraction (SCF) was greatest in cultures at an OTRmax of 30 mmol L-1 h(-1), reaching 92.9% for the wild type strain and 98.7% for the VHb-expressing strain, while no linearized pDNA was detected. This condition was replicated in a 1 L stirred tank bioreactor, and the results closely resembled those of cultures in MTPs.