Determinants in the sequence specific binding of two plant transcription factors, CBF1 and NtERF2, to the DRE and GCC motifs

Arabidopsis ERF proteins such as DREB1, DREB2, and CBF1 bind to the dehydration-responsive element (DRE), which has the sequence TACCGACAT. Mutation analyses reveal that a central 5 bp CCGAC core of the DRE is the minimal sequence motif (designated as the DRE motif in this paper), to which the ERF domain fragment of CBF1 (CBF1-F) binds specifically with a binding K-d at the nanomolar level. In contrast, the ERF domain fragment of the tobacco ERF2 (NtERF2-F) does not interact with the DRE motif, but restrictedly recognizes the sequence containing a minimal 6 bp GCCGCC motif (designated as the GCC motif in this paper). However, CBF1-F binds to the GCC motif with a binding activity similar to its binding activity for the DRE motif. These in vitro binding variations were further demonstrated through reporter cotransformation assays, suggesting that the DRE and GCC motifs are two similar sequence motifs sharing a common core region of CCGNC with a discriminating guanine base at the 5'-end of the GCC motif. Binding analyses with the mutated ERF domain show that such a unique binding of NtERF2-F to the GCC motif can be altered by the substitution of A14 with valine in beta-strand 2 of its ERF domain, the mutant NtERF2-F, ERFav, acquiring a binding to the DRE motif with a K-d comparable to that for CBF1-F binding to the DRE motif. This demonstrates that A14 is an important determinant of the NtERF2-F binding, specificity. A possible mechanism of the binding, specificity determination is discussed.