The fermentative alcohol dehydrogenase of Escherichia coli is encoded
by the adhE gene, which is induced under anaerobic conditions but repr
essed in air. Previous work suggested that induction of adhE might dep
end on NADH levels. We therefore directly measured the NAD(+) and NADH
levels for cultures growing aerobically and anaerobically on a series
of carbon sources whose metabolism generates different relative amoun
ts of NADH. Expression of adhE was monitored both by assay of alcohol
dehydrogenase activity and by expression of phi(adhE'-lacZ) gene fusio
ns. The expression of the adhE gene correlated with the ratio of NADH
to NAD(+) The role of NADH in eliciting adhE induction was supported b
y a variety of treatments known to change the ratio of NADH to NAD(+)
or alter the total NAD(+)-plus-NADH pool. Blocking the electron transp
ort chain, either by mutation or by chemical inhibitors, resulted in t
he artificial induction of the adhE gene under aerobic conditions. Con
versely, limiting NAD synthesis, by introducing mutational blocks into
the biosynthetic pathway for nicotinic acid, decreased the expression
of adhE under anaerobic conditions. This, in turn, was reversed by su
pplementation with exogenous NAD or nicotinic acid. In merodiploid str
ains carrying deletion or insertion mutations abolishing the synthesis
of AdhE protein, an adhE-lacZ fusion was expressed at nearly 10-fold
the level observed in an adhE(+) background. Introduction of mutant ad
hE alleles producing high levels of inactive AdhE protein gave results
equivalent to those seen in absence of the AdhE protein. This finding
implies that it is the buildup of NADH due to lack of enzyme activity
, rather than the absence of the AdhE protein per se, which causes inc
reased induction of the phi(adhE'-lacZ) fusion. Moreover, mutations gi
ving elevated levels of active AdhE protein decreased the induction of
the phi(adhE'-lacZ) fusion. This finding suggests that the enzymatic
activity of the AdhE protein modulates the level of NADH under anaerob
ic conditions, thus indirectly regulating its own expression.