Improved H+/O ratio and cell yield of Escherichia coli with genetically altered terminal quinol oxidases.

Escherichia coli wild-type cells, containing both cytochrome bo- and bd-type terminal oxidases, pumped protons with a H+/O ratio of 4.5-4.9 upon an oxygen pulse, while mutant cells, deprived of either cytochrome bo (deltacyo) or bd (deltacyd), showed values of 3.5-4.1 or 4.8-5.6, respectively. The cell yield of the cyo-less mutant was about 15% lower than that of the wild-type strain, while that of the cyd-less strain which over-produced cytochrome bo was about 10% higher than that of the wild-type. The simple cyd-less strain without over-production of cytochrome bo showed a high H+/O ratio, but its cell yield was low and variable from culture to culture. The growth inhibition and accelerated H+ permeability of the cell membrane of the latter strain seems due to the deletion of cytochrome bd (CydAB), the terminal oxidase having a very low K(m) for O2, which may result in severe stress on the cell. Over-production of cytochrome bo by as much as 0.4 nmol/mg membrane protein could compensate for this defect.