APETALA2 (AP2) plays an important role in the control of Arabidopsis f lower and seed development and encodes a putative transcription factor that is distinguished by a novel DNA binding motif referred to as the AP2 domain. In this study we show that the AP2 domain containing or RAP2 (related to AP2) family of proteins is encoded by a minimum of 12 genes in Arabidopsis. The RAP2 genes encode two classes of proteins, AP2-like and EREBP-like, that are defined by the number of AP2 domains in each polypeptide as well as by two sequence motifs referred to as the YRG and RAYD elements that are located within each AP2 domain. RAP2 genes are differentially expressed in f lower, leaf, inf lorescence stem, and root. Moreover, the expression of at least three RAP2 genes in vegetative tissues are controlled by AP2. Thus, unlike other f loral homeotic genes, AP2 is active during both reproductive and vegetative development.Genetic and molecular studies have defined an evolutionarily conserved network of genes that control flower development in Arabidopsis and in other plant species (1, 2). In Arabidopsis, the homeotic gene APETALA2 (AP2) has been shown to control three important processes during flower development: (i) the establishment of flower meristem identity (3, 4), (ii) the specification of flower organ identity and the regulation of floral organogenesis (5-9), and (iii) the temporal and spatial regulation of flower homeotic gene activity (10). Genetic studies have shown that AP2 is also required for normal ovule and seed development (9,11,12). Consistent with its genetic functions AP2 is expressed throughout flower development, in immature floral buds, in all four types of flower organ primordia, and in developing ovules and seeds (ref. 9; B. den Boer and K.D.J., unpublished data). Unlike most other flower homeotic genes, AP2 is also expressed at the mRNA level in both stem and leaf, suggesting that AP2 may also function during vegetative development (9). However, ap2 mutant plants show no dramatic defects during either stem or leaf development. One hypothesis to explain this apparent paradox is that the Arabidopsis genome may be genetically redundant for AP2 function during stem and leaf development.Previous studies have suggested that AP2 functions as a nuclear transcription factor in plant cells (9). DNA sequence analysis showed that AP2 encodes a theoretical polypeptide of 432 amino acids that is distinct from known fungal and animal regulatory proteins (9). One important feature of the AP2 protein is a novel 68-amino acid repeated motif called the AP2 domain. The AP2 domain has been shown to be essential for AP2 functions and contains an 18-amino acid core region that is predicted to form an amphipathic ␣-helix (9). In tobacco, four AP2-like AP2 domain containing proteins have been identified (13). These proteins, referred to as the ethylene responsive element binding proteins (EREBPs), are thought to regulate gene expression at the transcriptional level. In vitro studies have shown that the EREBP AP2 do...

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