Differential roles for menaquinone and demethylmenaquinone in anaerobic electron transport of E. coli and their fnr-independent expression.

Escherichia coli grown with glucose in the absence of added electron acceptors contained 3-4 times more naphthoquinones (menaquinone plus demethylmenaquinone) than in the presence of O2. Presence of electron acceptors resulted in a slight additional increase of the naphthoquinone content. A strain defective in the fnr gene, which encodes the transcriptional activator of anaerobic respiration, showed the same response. With fumarate or dimethyl sulfoxide present, 94% of the naphthoquinones consisted of menaquinone, while with nitrate up to 78% was demethylmenaquinone. With trimethylamine N-oxid as the acceptor the proportion was intermediate. From the donor substrates of anaerobic respiration only glycerol had a significant influence on the ratio of the contents of the 2 quinones. It is concluded that FNR, the gene product of the fnr gene, is not required for anaerobic derepression of naphthoquinone biosynthesis. Menaquinone appears to be involved specifically in the respiration with fumarate or dimethyl sulfoxide, and demethylmenaquinone in nitrate respiration. Both naphthoquinones appear to serve in trimethylamine N-oxide respiration.