The study investigates the protein transcription factor IIIA (TFIIIA) from Xenopus oocytes, which is essential for 5S RNA transcription. The 7S particle, containing 5S RNA and TFIIIA, is found to contain 7-11 zinc atoms, suggesting the presence of repetitive zinc-binding domains. Amino acid sequence analysis reveals nine tandem units, each consisting of approximately 30 residues, with invariant cysteine and histidine pairs as zinc ligands. These domains form the major part of the protein and are likely to be independently folded structures centered on a zinc ion. This structure explains how TFIIIA can bind to the long internal control region of the 5S RNA gene and remain bound during RNA polymerase passage. The study also discusses the evolutionary advantages of this repeating design and its potential role in DNA binding and transcription regulation.The study investigates the protein transcription factor IIIA (TFIIIA) from Xenopus oocytes, which is essential for 5S RNA transcription. The 7S particle, containing 5S RNA and TFIIIA, is found to contain 7-11 zinc atoms, suggesting the presence of repetitive zinc-binding domains. Amino acid sequence analysis reveals nine tandem units, each consisting of approximately 30 residues, with invariant cysteine and histidine pairs as zinc ligands. These domains form the major part of the protein and are likely to be independently folded structures centered on a zinc ion. This structure explains how TFIIIA can bind to the long internal control region of the 5S RNA gene and remain bound during RNA polymerase passage. The study also discusses the evolutionary advantages of this repeating design and its potential role in DNA binding and transcription regulation.