Members of the ARID3 subfamily in all species studied are characterized by the presence of an ‘extended’ ARID sequence, a region of very high identity (>70% identity across ∼35 residues) immediately following the core ARID consensus (see Figure 1). This region includes Helix 7, which is so far unique to the ARID3 subfamily, and extends beyond it. The extended ARID region has been identified as a DNA contact region in Dri (18). The extended ARID sequence is not present in the ARID5 subfamily, so cannot be a required determinant of sequence specificity. However, a corresponding position C-terminal to the core ARID consensus in MRF2 has been identified as a DNA contact region (17). In contrast, the corresponding region in p270 does not appear to make significant DNA contact (15). To assess the contribution of this region of Dri to sequence specificity, an in-frame deletion of sequence encoding 28 amino acids was generated in this region. The resulting mutant is designated Dri.ΔC. The DNA pull-down assay indicates that this construct retains a considerable measure of sequence selectivity (Figure 8). The construct shows slightly reduced DNA-binding affinity, consistent with the interpretation that the deleted region includes a DNA contact site.
