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Journal of Bacteriology, September 1999, p. 5303-5308, Vol. 181, No. 17
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
The napF and narG Nitrate
Reductase Operons in Escherichia coli Are Differentially
Expressed in Response to Submicromolar Concentrations of Nitrate but
Not Nitrite
Henian
Wang,
Ching-Ping
Tseng,
and
Robert P.
Gunsalus*
Department of Microbiology and Molecular
Genetics, University of California, Los Angeles, California 90095-1489
Received 8 July 1998/Accepted 17 June 1999
Escherichia coli synthesizes two biochemically distinct
nitrate reductase enzymes, a membrane-bound enzyme encoded by the narGHJI operon and a periplasmic cytochrome
c-linked nitrate reductase encoded by the
napFDAGHBC operon. To address why the cell makes these two
enzymes, continuous cell culture techniques were used to examine
napF and narG gene expression in response to
different concentrations of nitrate and/or nitrite. Expression of the
napF-lacZ and narG-lacZ reporter fusions in
strains grown at different steady-state levels of nitrate revealed that
the two nitrate reductase operons are differentially expressed in a
complementary pattern. The napF operon apparently encodes a
"low-substrate-induced" reductase that is maximally expressed only
at low levels of nitrate. Expression is suppressed under high-nitrate
conditions. In contrast, the narGHJI operon is only weakly
expressed at low nitrate levels but is maximally expressed when nitrate
is elevated. The narGHJI operon is therefore a
"high-substrate-induced" operon that somehow provides a second and
distinct role in nitrate metabolism by the cell. Interestingly,
nitrite, the end product of each enzyme, had only a minor effect on the
expression of either operon. Finally, nitrate, but not nitrite, was
essential for repression of napF gene expression. These
studies reveal that nitrate rather than nitrite is the primary signal
that controls the expression of these two nitrate reductase operons in
a differential and complementary fashion. In light of these findings,
prior models for the roles of nitrate and nitrite in control of
narG and napF expression must be reconsidered.
*
Corresponding author. Mailing address: Department of
Microbiology and Molecular Genetics, University of California, Los
Angeles, CA 90095-1489. Phone: (310) 206-8201. Fax: (310) 206-5231. E-mail: robg{at}microbiol.ucla.edu.

Present address: Institute of Biological Science and Technology,
National Chiao Tung University, Hsinchu 30050, Taiwan, Republic
of
China.
Journal of Bacteriology, September 1999, p. 5303-5308, Vol. 181, No. 17
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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