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J Bacteriol, May 1998, p. 2660-2669, Vol. 180, No. 10
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Roles of DctA and DctB in Signal Detection by the Dicarboxylic
Acid Transport System of Rhizobium leguminosarum
Colm J.
Reid
and
Philip S.
Poole*
School of Animal and Microbial Sciences,
University of Reading, Reading RG6 6AJ, United Kingdom
Received 16 October 1997/Accepted 6 March 1998
The dctA gene, coding for the dicarboxylate transport
protein, has an inducible promoter dependent on activation by the
two-component sensor-regulator pair DctB and DctD. LacZ fusion analysis
indicates that there is a single promoter for dctB and
dctD. The dctA promoter is also induced by
nitrogen limitation, an effect that requires DctB-DctD and NtrC. DctB
alone is able to detect dicarboxylates in the absence of DctA and
initiate transcription via DctD. However, DctA modifies signal
detection by DctB such that in the absence of DctA, the ligand
specificity of DctB is broader. dctAp also responds to
heterologous induction by osmotic stress in the absence of DctA. This
effect requires both DctB and DctD. A transposon insertion in the
dctA-dctB intergenic region (dctA101) which
locks transcription of dctA at a constitutive level
independent of DctB-DctD results in improper signalling by DctB-DctD.
Strain RU150, which carries this insertion, is defective in
nitrogen fixation (Fix
) and grows very poorly on
ammonia as a nitrogen source whenever the DctB-DctD signalling circuit
is activated by the presence of a dicarboxylate ligand. Mutation of
dctB or dctD in strain RU150
reinstates normal growth on dicarboxylates. This suggests that
DctD-P improperly regulates a heterologous nitrogen-sensing operon. Increased expression of DctA, either via a plasmid or by
chromosomal duplication, restores control of DctB-DctD and allows
strain RU150 to grow on ammonia in the presence of a dicarboxylate. Thus, while DctB is a sensor for dicarboxylates in its own right, it is
regulated by DctA. The absence of DctA allows DctB and DctD to become
promiscuous with regard to signal detection and cross talk with other
operons. This indicates that DctA contributes significantly to the
signalling specificity of DctB-DctD and attenuates cross talk with
other operons.
*
Corresponding author. Mailing address: School of Animal
and Microbial Sciences, University of Reading, Whiteknights, P.O. Box
228, Reading RG6 6AJ, United Kingdom. Phone: 44 118 9318895. Fax: 44 118 9316537. E-mail: p.s.poole{at}reading.ac.uk.

Present address: Institute of Molecular Medicine, University of
Oxford, John Radcliffe Hospital, Oxford OX3 9DU, United Kingdom.
J Bacteriol, May 1998, p. 2660-2669, Vol. 180, No. 10
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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