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Plant and Cell Physiology Advance Access originally published online on June 19, 2009
Plant and Cell Physiology 2009 50(8):1439-1448; doi:10.1093/pcp/pcp089
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© The Author 2009. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

A Synechocystis Homolog of SipA Protein, Ssl3451, Enhances the Activity of the Histidine Kinase Hik33

Tasuku Sakayori, Yoshihiro Shiraiwa and Iwane Suzuki*

Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki, 305-8572 Japan

*Corresponding author: E-mail, iwanes6803{at}biol.tsukuba.ac.jp; Fax, +81-29-853-4908.


  Abstract

In the cyanobacterium Synechocystis sp. PCC 6803, the histidine kinase Hik33 regulates the expression of several stress-inducible genes. Recently, a yeast two-hybrid screen revealed a specific interaction between Hik33 and a small protein, Ssl3451. To investigate the function of Ssl3451, we developed an assay to monitor the autophosphorylation of Hik33 in vitro. Addition of Ssl3451 to the reaction mixture dramatically enhanced the autophosphorylation activity of Hik33. Pulse–chase experiments revealed that Ssl3451 stimulated the autophosphorylation of Hik33 but did not affect its dephosphorylation. These findings indicated that Ssl3451 might be an activator of Hik33. When the amount of Hik33 was kept constant and the amount of Ssl3451 was increased in the reaction mixture, the extent of autophosphorylation of Hik33 reached a plateau when equimolar concentrations were present, suggesting that Ssl3451 enhances the activity of Hik33 by associating with it with a 1 : 1 stoichiometry. Disruption of the gene for Ssl3451 resulted in increased expression of the hliB gene, which is induced by Hik33 under standard growth conditions, but it did not affect the levels of the hliB mRNA at low temperature. Together, these results suggest that Ssl3451 might enhance the activity of Hik33 both in vitro and in vivo.

Keywords: Autophosphorylation - Cyanobacterium - Gene expression - Response regulator - Signal transduction - Two-component system

Abbreviations: EST, expressed sequence tag; Hik, histidine kinase; Rre, response regulator; TCS, two-component signal transduction system.

(Received March 11, 2009; Accepted June 17, 2009)
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