FUNCTIONAL DOMAINS OF THE FLORAL REGULATOR AGAMOUS - CHARACTERIZATIONOF THE DNA-BINDING DOMAIN AND ANALYSIS OF DOMINANT-NEGATIVE MUTATIONS

The Arabidopsis MADS box gene AGAMOUS (AG) controls reproductive organ identity and floral meristem determinacy. The AG protein binds in vit ro to DNA sequences similar to the targets of known MADS domain transc ription factors. Whereas most plant MADS domain proteins begin with th e MADS domain, AG and its orthologs contain a region N-terminal to the MADS domain. All plant MADS domain proteins share another region with moderate sequence similarity called the K domain. Neither the region (I region) that lies between the MADS and K domains nor the C-terminal region is conserved. We show here that the AG MADS domain and the I r egion are necessary and sufficient for DNA binding in vitro and that A G binds to DNA as a dimer. To investigate the in vivo function of the regions of AG not required for in vitro DNA binding, we introduced sev eral AG constructs into wild-type plants and characterized their flora l phenotypes. We show that transgenic Arabidopsis plants with a 35S-AG construct encoding an AG protein lacking the N-terminal region produc ed apetala2 (ap2)-like flowers similar to those ectopically expressing AG proteins retaining the N-terminal region. This result suggests tha t the N-terminal region is not required to produce the ap2-like phenot ype. In addition, transformants with a 35S-AG construct encoding an AG protein lacking the C-terminal region produced ag-like flowers, indic ating that this truncated AG protein inhibits normal AG function. Fina lly, transformants with a 35S-AG construct encoding an AG protein lack ing both K and C regions produced flowers with more stamens and carpel s. The phenotypes of the AG transformants demonstrate that both the K domain and the C-terminal region have important and distinct in vivo f unctions. We discuss possible mechanisms through which AG may regulate downstream genes.