A G-box-binding protein from soybean binds to the E1 auxin-response element in the soybean GH3 promoter and contains a proline-rich repression domain

The E1 promoter fragment (-249 to -203) is one of three auxin-response elements (AuxREs) in the soybean (Glycine max L.) GH3 promoter (Z.-B. Liu, T. Ulmasov, X. Shi, G. Hagen, T.J. Guilfoyle [1994] Plant Cell 6: 645-657). Results presented here further characterize and delimit the AuxRE within the E1 fragment. The E1 fragment functioned as an AuxRE in transgenic tobacco (Nicotiana fabacum L.) plants, as well as in transfected protoplasts. The AuxRE within E1 contains a C-box, and this G-box was used to clone a G-box-binding factor (CBF) from soybean (SGBF-2). This 45-kD GBF contains an N-terminal proline-rich domain and a C-terminal basic/leucine zipper DNA-binding domain. Gel-mobility shift assays were used to characterize the binding specificity of SGBF-2. Antiserum raised against recombinant SGBF-2 was used to further characterize SGBF-2 and antigenically related GBFs in soybean nuclear extracts. Co-transfection assays with effector and reporter plasmids in carrot (Daucus carota L.) protoplasts indicated that the N-terminal proline-rich domain of SGBF-2 functioned as a repression domain in both basal and auxin-inducible transcription.