Affiliation:
1. The School of Animal & Microbial Sciences, University of Reading, Reading RG6 6AJ,1 and
2. The Krebs Institute for Biomolecular Research, Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN,2 United Kingdom
Abstract
ABSTRACT
The
dcuA
and
dcuB
genes of
Escherichia coli
encode homologous proteins that appear to function as independent and mutually redundant C
4
-dicarboxylate transporters during anaerobiosis. The
dcuA
gene is 117 bp downstream of, and has the same polarity as, the aspartase gene (
aspA
), while
dcuB
is 77 bp upstream of, and has the same polarity as, the anaerobic fumarase gene (
fumB
). To learn more about the respective roles of the
dcu
genes, the environmental and regulatory factors influencing their expression were investigated by generating and analyzing single-copy
dcuA
- and
dcuB-lacZ
transcriptional fusions. The results show that
dcuA
is constitutively expressed whereas
dcuB
expression is highly regulated. The
dcuB
gene is strongly activated anaerobically by FNR, repressed in the presence of nitrate by NarL, and subject to cyclic AMP receptor protein (CRP)-mediated catabolite repression. In addition,
dcuB
is strongly induced by C
4
-dicarboxylates, suggesting that
dcuB
is under the control of an uncharacterized C
4
-dicarboxylate-responsive gene regulator. Northern blotting confirmed that
dcuA
(and
aspA
) is expressed under both aerobic and anaerobic conditions and that
dcuB
(and
fumB
) is induced anaerobically. Major monocistronic transcripts were identified for
aspA
and
dcuA
, as well as a minor species possibly corresponding to an
aspA-dcuA
cotranscript. Five major transcripts were observed for
dcuB
and
fumB
: monocistronic transcripts for both
fumB
and
dcuB
; a
dcuB-fumB
cotranscript; and two transcripts, possibly corresponding to
dcuB-fumB
and
fumB
mRNA degradation products. Primer extension analysis revealed independent promoters for
aspA
,
dcuA
, and
dcuB
, but surprisingly no primer extension product could be detected for
fumB
. The expression of
dcuB
is entirely consistent with a primary role for DcuB in mediating C
4
-dicarboxylate transport during anaerobic fumarate respiration. The precise physiological purpose of DcuA remains unclear.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
Cited by
77 articles.
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