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J Bacteriol, July 1998, p. 3563-3569, Vol. 180, No. 14
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Proposed Signal Transduction Role for Conserved
CheY Residue Thr87, a Member of the Response Regulator
Active-Site Quintet
Jeryl L.
Appleby and
Robert B.
Bourret*
Department of Microbiology and Immunology,
University of North Carolina, Chapel Hill, North Carolina 27599-7290
Received 2 February 1998/Accepted 7 May 1998
CheY serves as a structural prototype for the response regulator
proteins of two-component regulatory systems. Functional roles have
previously been defined for four of the five highly conserved residues
that form the response regulator active site, the exception being the
hydroxy amino acid which corresponds to Thr87 in CheY. To investigate
the contribution of Thr87 to signaling, we characterized, genetically
and biochemically, several cheY mutants with amino acid
substitutions at this position. The hydroxyl group appears to be
necessary for effective chemotaxis, as a Thr
Ser substitution was the
only one of six tested which retained a Che+ swarm
phenotype. Although nonchemotactic, cheY mutants with amino acid substitutions T87A and T87C could generate clockwise flagellar rotation either in the absence of CheZ, a protein that stimulates dephosphorylation of CheY, or when paired with a second site-activating mutation, Asp13
Lys, demonstrating that a hydroxy amino acid at position 87 is not essential for activation of the flagellar switch. All purified mutant proteins examined phosphorylated efficiently from
the CheA kinase in vitro but were impaired in autodephosphorylation. Thus, the mutant CheY proteins are phosphorylated to a greater degree
than wild-type CheY yet support less clockwise flagellar rotation. The
data imply that Thr87 is important for generating and/or stabilizing
the phosphorylation-induced conformational change in CheY. Furthermore,
the various position 87 substitutions differentially affected several
properties of the mutant proteins. The chemotaxis and
autodephosphorylation defects were tightly linked, suggesting common
structural elements, whereas the effects on self-catalyzed and
CheZ-mediated dephosphorylation of CheY were uncorrelated, suggesting
different structural requirements for the two dephosphorylation
reactions.
*
Corresponding author. Mailing address: Department of
Microbiology and Immunology, University of North Carolina, Chapel Hill, NC 27599-7290. Phone: (919) 966-2679. Fax: (919) 962-8103. E-mail: bourret{at}med.unc.edu.
J Bacteriol, July 1998, p. 3563-3569, Vol. 180, No. 14
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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