Genetic analysis of the signal-sensing region of the histidine protein kinase VirA of Agrobacterium tumefaciens |
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Authors: | Toyoda-Yamamoto A Shimoda N Machida Y |
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Institution: | (1) Division of Biological Science, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602 Japan E-mail: yas@bio.nagoya-u.ac.jp Tel. +81-52-789-2502; Fax: +81-52-789-2966, JP |
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Abstract: | The membrane-bound sensor protein kinase VirA of Agrobacterium tumefaciens detects plant phenolic substances, which induce expression of vir genes that are essential for the formation of the crown gall tumor. VirA also responds to specific monosaccharides, which
enhance vir expression. These sugars are sensed by the periplasmic domain of VirA that includes the region homologous to the chemoreceptor
Trg, and the phenolics are thought to be detected by a part of the cytoplasmic linker domain, while the second transmembrane
domain (TM2) is reported to be nonessential. To define regions of VirA that are essential for signal sensing, we introduced
base-substitution and deletion mutations into coding regions that are conserved among the respective domains of VirA proteins
from various Agrobacterium strains, and examined the effects of these mutations on vir induction and tumorigenicity. The results show that the Trg-homologous region in the periplasmic domain is not essential
for the enhancement of vir gene expression by sugars. Most mutations in the TM2 domain also failed to influence enhancement by sugars and reduced the
level of vir induction, but a mutation in the TM2 region adjacent to the cytoplasmic linker abolished induction of the vir genes. In the linker domain, sites essential for vir induction by phenolics were scattered over the entire region. We propose that a topological feature formed by the linker
domain and at least part of the TM2 may be crucial for activation of a membrane-anchored VirA protein. Complementation analysis
with two different VirA mutants suggested that intermolecular phosphorylation between VirA molecules occurs in vivo, and that
two intact periplasmic regions in a VirA dimer are required for the enhancement of vir induction by sugars.
Received: 14 December 1999 / Accepted: 10 April 2000 |
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Keywords: | Agrobacterium tumefaciens Histidineprotein kinase VirA Expression of vir genes Signal transduction Intermolecular phosphorylation |
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