DNA-Binding Specificity of NGFI-A and Related Zinc Finger Transcription Factors

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Abstract
NGFI-A is the prototypic member of a family of immediate-early gene-encoded transcription factors which includes NGFI-C, Egr3, and Krox20. These proteins possess highly homologous DNA-binding domains, composed of three Cys2-His2 zinc fingers, and all bind to and activate transcription from the sequence GCGGGGGCG. We used a PCR-mediated random site selection protocol to determine whether other sites could be bound by these proteins and the extent to which their binding site preferences are similar or different. The high-affinity consensus sites generated from the selection data are similar, and the combined consensus sequence is T-G-C-G-T/g-G/A-G-G-C/a/t-G-G/T (lowercase letters indicate bases selected less frequently). Using gel shift assays, we found that sequences that diverge from the consensus were bound by NGFI-A, confirming that there is greater variability in binding sites than has generally been acknowledged. We also provide evidence that protein-DNA interactions not noted, or whose importance was not apparent from the X-ray cocrystal structure of the NGFI-A zinc fingers complexed with DNA, contribute significantly to the binding energy of these proteins and confirm that an optimal site is at least 10 instead of 9 nucleotides in length. In contrast to the similarities in binding specificity among these proteins we found that while NGFI-A, Egr3, and Krox20 have comparable DNA binding affinities and kinetics of dissociation, the affinity of NGFI-C is more than threefold lower. This could result in differential regulation of target genes in cells where NGFI-C and the other proteins are coexpressed. Furthermore, we show that this affinity difference is a property not of the zinc fingers themselves but rather of the protein context of the DNA-binding domain.