Characterization and expression of a new class of zinc finger protein that binds to silencer region of ascorbate oxidase gene

A unique A/T-rich sequence (5'-AAAAAGTAAAAA-GTAAAAAAGTAAAAAG-3), referred to as the AGTA repeat, is found in the silencer region of the pumpkin ascorbate oxidase gene. A cDNA for protein (AOBP) that binds to the AGTA repeat was isolated from pumpkin by the southwestern method. The AOBP protein has a new class of zinc/DNA-binding domain named Dof/MOA domain that is highly conserved in many plant proteins and is significantly related to those of steroid hormone receptors and GATA1. Gel retardation analysis indicated that AOBP bound to the AGTA repeat through the Dof/MOA domain. Metal chelators, 1,10-phenanthroline and EDTA, specifically inhibited the DNA binding of AOBP, indicating that metal coordination plays an important role in DNA binding of AOBP. Thus, the Dof/MOA domain acts as a zinc/DNA-binding domain in AOBP. Gel retardation analysis with mutated oligonucleotides suggested that the Dof/MOA domain recognized the AGTA core sequence. AOBP mRNA was expressed in mature tissues of pumpkin, but was expressed only in small amounts or was not expressed in growing tissues. Furthermore, the expression was auxin-independent. The expression pattern of AOBP and that of ascorbate oxidase did not show a positive correlation.