A novel DNA-binding domain that may form a single zinc finger motif

MNB1a is a DNA-binding protein from maize that interacts with the 35S promoter of cauliflower mosaic virus. This protein did not show significant homologies with any other DNA-binding protein and MNB1a seemed to be a member of a multigene family. In this study, isolation of cDNAs from the gene family to which MNB1a belongs revealed a unique conserved domain, referred to herein as the Dof domain, that contains a novel cysteine-rich motif for a single putative zinc finger. The amino acid sequence of the Dof domain and the arrangement of cysteine residues in this domain differ from those of known zinc finger motifs. However, the Dof domain was shown to be a DNA-binding domain that required Zn2+ ions for activity. Mutations at cysteine residues eliminated the DNA-binding activity of MNB1a. Thus, the Dof domain may be classified as a novel zinc finger motif. In addition, Southern blot analysis and a survey of DNA databases suggested that proteins that include Dof domains might exist in other eukaryotes, at least in the plant kingdom.