1H, 15N and 13C resonance assignments for the first three zinc fingers of transcription factor IIIA.

The first three zinc fingers (ZF1-3) of transcription factor IIIA (TFIIIA) from Xenopus have been shown to contribute the majority of the binding energy to the intact TFIIIA-DNA interaction [Liao et al. (1992) J. Mol. Biol., 223, 857-871]. We have expressed a 92-amino acid polypeptide containing the three N-terminal zinc fingers of TFIIIA. This three-fingered polypeptide has been isotopically labeled with 15N and 13C in E. coli and purified to homogeneity. Assignment of backbone 1H, 15N, aliphatic 1H and 13C and aromatic 1H and 13C resonances of delta NZF1-3 has been obtained using a combination of single-, double- and triple-resonance multidimensional NMR experiments. The secondary structures for each finger have been determined from NOE connectivities, 3JNH alpha values and chemical shifts. The results show that each finger folds into a canonical beta-sheet-helix zinc finger structural motif, while the linkers adopt an extended structure. The helix between the two histidine ligands in ZF3 is distorted by zinc coordination, to accommodate the presence of four intervening amino acids instead of three as in ZF1 and ZF2.