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Identification and Molecular Characterization of ZAG1, the Maize Homolog of the Arabidopsis Floral Homeotic Gene AGAMOUS
Recent genetic and molecular studies in Arabidopsis and Antirrhinum suggest that mechanisms controlling floral development are well conserved among dicotyledonous species. To assess whether similar mechanisms also operate in more distantly related monocotyledonous species, we have begun to clone homologs of Arabidopsis floral genes from maize. Here we report the characterization of two genes, designated ZAG1 and ZAG2 (for Zea AG), that were cloned from a maize inflorescence cDNA library by low stringency hybridization with the AGAMOUS (AG) cDNA from Arabidopsis. ZAG1 encodes a putative polypeptide of 286 amino acids having 61% identity with the AGAMOUS (AG) protein. Through a stretch of 56 amino acids, constituting the MADS domain, the two proteins are identical except for two conservative amino acid substitutions. The ZAG2 protein is less similar to AG, with 49% identity overall and substantially less similarity than ZAG1 outside the well-conserved MADS domain. Like AG, ZAG1 RNA accumulates early in stamen and carpel primordia. In contrast, ZAG2 expression begins later and is restricted to developing carpels. Hybridization to genomic DNA with the full-length ZAG1 cDNA under moderately stringent conditions indicated the presence of a large family of related genes. Mapping data using maize recombinant inbreds placed ZAG1 and ZAG2 near two loci that are known to affect maize flower development, Polytypic ear (Pt) and Tassel seed4 (Ts4), respectively. The ZAG1 protein from in vitro translations binds to a consensus target site that is recognized by the AG protein. These data suggest that maize contains a homolog of the Arabidopsis floral identity gene AG and that this gene is conserved in sequence and function.