The conserved KNOX domain mediates specificity of tobacco KNOTTED1-type homeodomain proteins

Overproduction of the tobacco KNOTTED1-type homeodomain proteins NTH1, NTH1 5, and NTH23 in transgenic tobacco plants causes mild, severe, and no morph ological alterations, respectively. The deduced amino acid sequences of the homeodomains and adjacent ELK domains are highly conserved, and the N-term inal KNOX domains also are moderately conserved. To investigate the contrib utions of both the conserved and divergent regions to the severity of morph ological alterations, we generated chimeric proteins by exchanging differen t regions of NTH1, NTH15, and NTH23. The severity of the abnormal phenotype was dependent upon the synergistic action of both the N terminus, containi ng the KNOX domain, and the C terminus, containing the ELK homeodomain. Det ailed analysis focusing on the C terminus revealed that the C-terminal half of the ELK domain is more effective in inducing the abnormal phenotypes th an are the homeodomains. Fcr the N terminus, severe morphological alteratio ns were induced by exchanging a part of the KNOX domain of NTH1 with the co rresponding region of NTH15. This limited region in the KNOX domain of all homeodomain proteins includes a predicted alpha-helical region, but only th at in NTH15 is predicted to form a typical amphipathic structure. We discus s the possibility, based on these results, that the secondary structure of the KNOX domain is important for the induction of abnormal morphology in tr ansgenic tobacco plants.