A Novel Family of Plant DNA-Binding Proteins Containing both HMG-Box and AT-Rich Interaction Domains

The A/T-rich interaction domain (ARID) and the HMG-box domain represent DNA-interaction modules that are found in sequence-specific as well as nonsequence-specific DNA-binding proteins. Both domains are found in a variety of DNA-interacting proteins in a wide range of eukaryotic organisms. Proteins that contain both an ARID and an HMG-box domain, here termed ARID-HMG proteins, appear to be specific for plants. This protein family is conserved in higher plants (both mono- and dicot plants) as well as lower plants such as the moss Physcomitrella. Since ARID-HMG proteins have not been studied experimentally, we have examined here two family members from Arabidopsis. The genes encoding ARID-HMG1 and ARID-HMG2 are widely expressed in Arabidopsis but at different levels. Subcellular localization experiments studying ARID-HMG1 and ARID-HMG2 fused to GFP by fluorescence microscopy show that both proteins localize primarily to cell nuclei. Analyses of the DNA-binding properties using electrophoretic mobility shift assays revealed that mediated by the HMG-box domain, ARID-HMG I binds structure specifically to DNA minicircles. Mediated by the ARID, the protein binds preferentially to A/T-rich DNA, when compared with G/C-rich DNA. Therefore, both DNA-binding domains contribute to the DNA interactions of ARID-HMGI. Accordingly, the protein combines DNA-binding properties characteristic of ARID and HMG-box proteins.