Transcription of the
cobinamide biosynthetic genes (the CobI operon) was induced under three different physiological conditions: anaerobiosis (anaerobic respiration or fermentation), aerobic respiration at low
oxygen levels, and aerobic respiration with a partial block of the electron transport chain. After a shift to inducing conditions, there was a time lag of approximately 50 min before the onset of CobI induction. Under conditions of anaerobic respiration, the level of CobI transcription was dependent on the nature of both the electron donor (
carbon and energy source) and the acceptor. Cells grown with electron acceptors with a lower midpoint potential showed higher CobI expression levels. The highest level of CobI transcription observed was obtained with
glycerol as the
carbon source and
fumarate as the electron acceptor. The high induction seen with
glycerol was reduced by mutational blocks in the
glycerol catabolic pathway, suggesting that
glycerol does not serve as a gratuitous inducer but must be metabolized to stimulate CobI transcription. In the presence of
oxygen, CobI operon expression was induced 6- to 20-fold by the following: inhibition of
cytochrome o oxidase with
cyanide, mutational blockage of
ubiquinone biosynthesis, and
starvation of mutant cells for
heme. We suggest that the CobI operon is induced in response to a reducing environment within the cell and not by the absence of
oxygen per se.