Molecular Basis for Specific Recognition of Both RNA and DNA by a Zinc Finger Protein

Transcription factor IIIA (TFIIIA) from Xenopus oocytes binds both the internal control region of the 5 S ribosomal RNA genes and the 5 S RNA transcript itself. The nucleic acid binding domain of TFIIIA contains nine tandemly repeated zinc finger motifs. A series of precisely truncated forms of this protein have been constructed and assayed for 5 S RNA and DNA binding. Different sets of zinc fingers were found to be responsible for high affinity interactions with RNA and with DNA. These results explain how a single protein can exhibit equal affinities for these two very different nucleic acids.