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It has been reported that expression of the ERF genes including AtEBP is regulated by AP2 activity (Okamuro et al., 1997). This observation lead to the investigation of the transcriptional regulation of AP2/EREBP domain-containing genes in the
current study. To investigate the relationships in the transcriptional regulation of AP2 and AtEBP, an analysis was made of the expression patterns of AP2 and ERF genes such as AtEBP and AtERF1 in different tissues (flowers, stems and leaves). To avoid cross-hybridization among AP2, AtEBP and AtERF1, each specific probe was used for Northern blot hybridization (Fig. 1). In the wild type (WT), the AtEBP mRNA level was high in leaves and low in flowers, while AP2 mRNA levels were low in all the tissues analysed (flowers, stems and leaves). There is a point-mutation in the AP2/EREBP
domain of AP2 (residue Gly-159 to Glu) in the ap2-5 mutant, which leads to reduced transcriptional activity of AP2 (Jofuku et al., 1994). As a result, the floral homeotic phenotype was observed. To test whether AP2 activity affects the AtEBP expression, we investigated mRNA accumulation of AtEBP in ap2-5. In ap2-5, the AtEBP mRNA level was increased in flowers, leaves and stems. The AP2 mRNA level of ap2-5 was also increased, especially in flowers and leaves, suggesting that AP2 activity also suppresses its own AP2 gene expression. The expression pattern of AtERF1 was similar to that of AtEBP: the AtERF1 mRNA level of ap2-5 increased in flowers, leaves and stems compared with the WT. mRNA accumulation of these genes was examined in ap2-7, and similar results were obtained (data not shown). These results may suggest that AP2 represses the expression of ERF genes such as AtEBP, AtERF1 and the AP2 gene itself.
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