Journal of Bacteriology, November 2000, p. 6279-6286, Vol. 182, No. 22
Department of Microbiology, School of
Medicine, University of Virginia, Charlottesville, Virginia 22908-0734
Received 12 June 2000/Accepted 22 August 2000
The histidine kinase (HK) component of many two-component
regulatory systems exhibits regulated ability to phosphorylate itself and to participate in transfer of phosphate to and from its cognate response regulator. The signaling system that controls expression of
the UhpT sugar phosphate transporter in Escherichia coli in response to external glucose 6-phosphate includes the HK protein UhpB
and the polytopic membrane protein UhpC, a UhpT homolog which is
required for responsiveness to an inducer and activation of UhpB. The
existence of a UhpBC signaling complex is suggested by the requirement
for UhpC for the activity of certain constitutively active variants of
UhpB, the dominance and epistasis relationships of uhp
alleles, and the finding that expression of UhpB in excess of UhpC has
a strong dominant-negative effect. Expression of a hybrid protein
containing the cytoplasmic C-terminal half of UhpB fused to glutathione
S-transferase (GST) also interfered with Uhp signaling.
This interference phenotype could not result solely from the
phosphatase activity of UhpB, because interference affected both
overexpressed UhpA and UhpA variants which are active in the absence of
phosphorylation. Variant forms of UhpB which were active in the absence
of UhpC carried amino acid substitutions near motifs conserved in HK
proteins. The GST fusion protein inhibited the ability of UhpA to bind
and activate transcription at the uhpT promoter. Unlike the
wild-type situation, a GST fusion variant carrying one of the
UhpB-activating substitutions, R324C, displayed autokinase activity and
phosphate transfer to UhpA but retained the ability to sequester UhpA
when it was altered in the conserved residues important for phosphate
transfer. Thus, the default state of UhpB is kinase off, and activation
of its phosphate transfer activity requires either the action of UhpC
or the occurrence of certain mutations in UhpB. The interference
phenotype shown by UhpB in excess of UhpC appears to include the
binding and sequestration of UhpA.
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Copyright © 2000, American Society for Microbiology. All rights reserved.
The Histidine Kinase Domain of UhpB Inhibits UhpA
Action at the Escherichia coli uhpT Promoter
and
*
Corresponding author. Mailing address: Department of
Microbiology, University of Virginia School of Medicine, P.O. Box
800734, Charlottesville, VA 22908-0734. Phone: (804) 924-2532. Fax:
(804) 982-1071. E-mail: rjk{at}virginia.edu.
Present address: 1181 Lamont Dr., Winston-Salem, NC 27103.
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