Journal of Bacteriology, December 1998, p. 6586-6596, Vol. 180, No. 24
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
The School of Animal & Microbial Sciences,
Received 24 August 1998/Accepted 13 October 1998
The dcuA and dcuB genes of
Escherichia coli encode homologous proteins that appear to
function as independent and mutually redundant
C4-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 C4-dicarboxylates, suggesting that
dcuB is under the control of an uncharacterized
C4-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
C4-dicarboxylate transport during anaerobic fumarate respiration. The precise physiological purpose of DcuA remains unclear.
*
Corresponding author. Mailing address: The School of
Animal & Microbial Sciences, University of Reading, Whiteknights,
P. O. Box 228, Reading RG6 6AJ, United Kingdom. Phone:
118-987-5123, ext. 7045 or 7886. Fax: 118-931-0180. E-mail:
S.C.Andrews{at}reading.ac.uk.
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