DOMAIN EXCHANGES BETWEEN KNAT3 AND KNAT1 SUGGEST SPECIFICITY OF THE KN1-LIKE HOMEODOMAINS REQUIRES SEQUENCES OUTSIDE OF THE 3RD HELIX AND N-TERMINAL ARM OF THE HOMEODOMAIN

Domain exchange constructs that traded regions surrounding the homeodo main were constructed for two kn1-like genes, KNAT1 and KNAT3, and int roduced into Arabidopsis thaliana under the control of the 35S CaMV pr omoter. The kn1-like homeodomain proteins all have the homeodomain loc ated near the C-terminus of the protein, and also share a second conse rved domain (the ELK domain) immediately N-terminal to the homeodomain . Progeny were scored for the appearance of the KNAT1 overexpression p henotype. A construct containing the KNAT3 N-terminus and the KNAT1 EL K- and homeodomain resulted in a KNAT1 overexpression phenotype, indic ating that specificity mainly resides within the ELK- and homeodomain region. Further exchanges demonstrated that specificity probably does not arise from a single region within the ELK and/or homeodomain but r ather requires sequences both N-terminal and C-terminal to residue 23 of the homeodomain. Further, in contrast to some animal homeodomains, KNAT1 does not utilize the residues of the N-terminal arm of the homeo domain far specificity.