Transacting factors often form homo- and heterodimers and regulate various targets, the type of regulation depending on the dimeric combination. The WUS and TALE subfamilies are two atypical homeodomains in plants. A homeodomain mediates sequence-specific binding to its target DNA and usually consists of 60 amino acid residues, whereas atypical homeodomains have extra amino acid residues in the well-conserved region. The genes OsWUS and OsPRS, which encode atypical homeodomain proteins from the WUS subfamily, and OsBEL and OSH15, which encode those from the TALE subfamily, were isolated from rice and tested for their interactions by yeast two-hybrid analysis. OsWUS and OsPRS formed homodimers and formed heterodimers with each other but did not form dimers with the TALE family homeodomain proteins OSH15 or OsBEL. Likewise, OSH15 and OsBEL formed homodimers and heterodimers but did not form dimers with the WUS family homeodomain proteins OsWUS and OsPRS. These findings suggest that the combinations of dimers are well correlated with the classification of these proteins on the basis of sequence similarity. RT-PCR analysis revealed that expression of OsWUS and OsPRS was detected in the same organs, namely floral buds, roots, and suspension cells. Therefore, it is possible that the proteins encoded by both of these genes function as homo- and heterodimers in planta. These results suggest that, during the evolution of these subfamilies, various combinations of dimers within proteins encoded by paralogous genes were formed and generated independent regulatory networks that enabled complex patterns of plant development.
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