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Plant and Cell Physiology Advance Access originally published online on October 23, 2007
Plant and Cell Physiology 2007 48(12):1809-1814; doi:10.1093/pcp/pcm145
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© The Author 2007. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Short Communication

Identification of Amino Acid Substitutions that Render the Arabidopsis Cytokinin Receptor Histidine Kinase AHK4 Constitutively Active

Kumiko Miwa, Kuniko Ishikawa, Kazunori Terada, Hisami Yamada, Tomomi Suzuki, Takafumi Yamashino* and Takeshi Mizuno

Laboratory of Molecular Microbiology, School of Agriculture, Nagoya University, Furocho, Chikusa-ku, Nagoya, 464-8601 Japan

*Corresponding author: E-mail, yamasino{at}agr.nagoya-u.ac.jp; Fax, +81-52-789-4091.


  Abstract

In Arabidopsis, three genes (AHK2, AHK3 and AHK4/CRE1) encode histidine kinases (His-kinases), which serve as cytokinin receptors. To understand how the external cytokinin signal activates the His-kinase across the cell membrane, we exploited the power of microbial genetics to isolate several AHK4 mutants that function independently of cytokinin in both prokaryotic and eukaryotic assay systems. In each mutant, a single amino acid substitution within the second membrane-spanning segment, or within the region around the phosphorylation His site, renders the His-kinase constitutively active. These mutant receptors appear to have a ‘locked-on’ conformation, even in the absence of stimulus. We discuss the implications of these data for the structure and function of the cytokinin receptor His-kinases in plants.

Keywords: Cytokinin assay system with microbial cells - Cytokinin receptor - His–Asp phosphorelay signaling - His-kinase - Structure and function

Abbreviations: His-kinase, histidine kinase; iP, isopentenyl adenine; X-Gal, 5-bromo-4-chloro-3-indolyl-β-D-galactoside.

(Received September 27, 2007; Accepted October 20, 2007)
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