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J. Biol. Chem., Vol. 281, Issue 49, 37868-37876, December 8, 2006

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A New Class of Signal Transducer in His-Asp Phosphorelay Systems^*

Shin-ichi Maeda¹, Chieko Sugita, Mamoru Sugita, and Tatsuo Omata

From the Laboratory of Molecular Plant Physiology, Graduate School of Bioagricultural Sciences and the Center for Gene Research, Nagoya University, Furocho, Chikusaku, Nagoya 464-8601, Japan

Nitrate transport activity of the LtnT permease of the cyanobacterium Synechococcus elongatus is activated when LtnA, a response regulator without an effector domain, is phosphorylated by LtnB, a hybrid histidine kinase. We identified a protein (LtnC) that is required for activation of LtnT. LtnC consists of an N-terminal histidine-containing phosphoacceptor (HisKA) domain, a receiver domain, and a unique C-terminal domain found in some cyanobacterial proteins. Because LtnC lacks an ATP-binding kinase domain of a histidine kinase, it is incapable of autophosphorylation, but LtnC is phosphorylated by LtnA. The histidine residue in the HisKA domain but not the aspartate residue in the receiver domain is essential for phosphorylation of LtnC and activation of LtnT. LtnC phosphorylation leads to oligomerization of the protein. Fusion of the C-terminal domain of LtnC to glutathione S-transferase, which forms oligomers, also activates LtnT, suggesting that oligomerization of the LtnC C-terminal domain causes LtnT activation. These results indicate that the C-terminal domain of LtnC acts as an effector domain that directs the output of the signal from the phosphorelay system. The two-step (His-Asp-His) phosphorelay system, composed of the LtnB, LtnA, and LtnC proteins, is distinct from the known phosphorelay systems, namely, the typical two-component system (His-Asp) and the multistep phosphorelay system (His-Asp-His-Asp), because the HisKA domain of LtnC is the terminal phosphoacceptor that determines the signal output. LtnC is a new class of signal transducer in His-Asp phosphorelay systems that contains a HisKA domain and an effector domain.

Received for publication, September 8, 2006 , and in revised form, October 11, 2006.

^* This work was supported by a Grant-in-aid for Scientific Research in Priority Areas (13206027) and in part by the 21st Century COE Program and a Grant-in-aid for Scientific Research in Priority Areas (18056008) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. S1.

¹ To whom correspondence should be addressed: Tel.: 81-52-789-4104; Fax: 81-52-789-4107; E-mail: maeda{at}agr.nagoya-u.ac.jp.

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