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Plant Cell, Vol. 11, 1419-1432, August 1999, Copyright © 1999, American Society of Plant Physiologists
The Conserved KNOX Domain Mediates Specificity of Tobacco KNOTTED1-Type Homeodomain Proteins
Tomoaki Sakamotoa,b,
Asuka Nishimurac,
Masanori Tamaokic,
Masako Kubac,
Hiroshi Tanakab,
Shuichi Iwahoria, and
Makoto Matsuokac
a Institute of Agriculture and Forestry, University of Tsukuba, Tsukuba, Ibaraki 305-0006, Japan
b National Institute of Agrobiological Resources, Tsukuba, Ibaraki 305-0856, Japan
c BioScience Center, Nagoya University, Chikusa, Nagoya 464-0814, Japan
Correspondence to:
Makoto Matsuoka, j45751a{at}nucc.cc.nagoya-u.ac.jp (E-mail), 81-52-789-5226 (fax)
Overproduction of the tobacco KNOTTED1-type homeodomain proteins NTH1, NTH15, and NTH23 in transgenic tobacco plants causes mild, severe, and no morphological alterations, respectively. The deduced amino acid sequences of the homeodomains and adjacent ELK domains are highly conserved, and the N-terminal KNOX domains also are moderately conserved. To investigate the contributions of both the conserved and divergent regions to the severity of morphological alterations, we generated chimeric proteins by exchanging different regions of NTH1, NTH15, and NTH23. The severity of the abnormal phenotype was dependent upon the synergistic action of both the N terminus, containing the KNOX domain, and the C terminus, containing the ELK homeodomain. Detailed analysis focusing on the C terminus revealed that the C-terminal half of the ELK domain is more effective in inducing the abnormal phenotypes than are the homeodomains. For the N terminus, severe morphological alterations were induced by exchanging a part of the KNOX domain of NTH1 with the corresponding region of NTH15. This limited region in the KNOX domain of all homeodomain proteins includes a predicted  -helical region, but only that in NTH15 is predicted to form a typical amphipathic structure. We discuss the possibility, based on these results, that the secondary structure of the KNOX domain is important for the induction of abnormal morphology in transgenic tobacco plants.
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