The 7S particle of Xenopus laevis oocytes contains 5S RNA and a 40-K protein, which is essential for 5S RNA transcription in vitro. Proteolytic digestion of the protein yields periodic intermediates spaced at 3-K intervals and a limit digest containing 3-K fragments. The native particle contains 7–11 zinc atoms, suggesting the protein has repetitive zinc-binding domains. Analysis of the amino acid sequence reveals nine tandem units, each about 30 residues long, containing two invariant cysteine and histidine pairs, common zinc ligands. These domains, each centered on a zinc ion, form the major part of the protein. This structure explains how the small protein can bind to the long internal control region of the 5S RNA gene and remain bound during RNA polymerase passage. The 5S RNA genes of Xenopus laevis, transcribed by RNA polymerase III, have been studied extensively. The 40-K protein, TFIIIA, binds to the internal control region of the 5S gene and is involved in autoregulation of 5S gene transcription. TFIIIA is part of the 7S ribonucleoprotein particle, which contains 5S RNA and TFIIIA. The particle requires zinc for stability, with a zinc-to-particle ratio of about 7.0 ± 0.5 mol Zn per mol particle. The protein contains nine repeated units, each with a zinc-binding site, and these units are arranged linearly. The sequence analysis shows that the protein has a highly repetitive structure, with each unit containing two invariant cysteine and histidine residues that coordinate zinc ions. The structure of TFIIIA suggests that it has a compact, linear arrangement of small domains, each centered on a zinc ion, which may explain its ability to bind to the long internal control region of the 5S RNA gene. The protein's structure is similar to other zinc-binding proteins, such as calmodulin and bacterial ferredoxin, but differs in its unique arrangement of cysteine and histidine residues. The study suggests that the repetitive structure of TFIIIA may have evolved through gene duplication or gene conversion, allowing it to perform multiple functions in the 5S gene control system. The protein's structure and function are important for understanding the regulation of 5S RNA transcription in Xenopus oocytes.The 7S particle of Xenopus laevis oocytes contains 5S RNA and a 40-K protein, which is essential for 5S RNA transcription in vitro. Proteolytic digestion of the protein yields periodic intermediates spaced at 3-K intervals and a limit digest containing 3-K fragments. The native particle contains 7–11 zinc atoms, suggesting the protein has repetitive zinc-binding domains. Analysis of the amino acid sequence reveals nine tandem units, each about 30 residues long, containing two invariant cysteine and histidine pairs, common zinc ligands. These domains, each centered on a zinc ion, form the major part of the protein. This structure explains how the small protein can bind to the long internal control region of the 5S RNA gene and remain bound during RNA polymerase passage. The 5S RNA genes of Xenopus laevis, transcribed by RNA polymerase III, have been studied extensively. The 40-K protein, TFIIIA, binds to the internal control region of the 5S gene and is involved in autoregulation of 5S gene transcription. TFIIIA is part of the 7S ribonucleoprotein particle, which contains 5S RNA and TFIIIA. The particle requires zinc for stability, with a zinc-to-particle ratio of about 7.0 ± 0.5 mol Zn per mol particle. The protein contains nine repeated units, each with a zinc-binding site, and these units are arranged linearly. The sequence analysis shows that the protein has a highly repetitive structure, with each unit containing two invariant cysteine and histidine residues that coordinate zinc ions. The structure of TFIIIA suggests that it has a compact, linear arrangement of small domains, each centered on a zinc ion, which may explain its ability to bind to the long internal control region of the 5S RNA gene. The protein's structure is similar to other zinc-binding proteins, such as calmodulin and bacterial ferredoxin, but differs in its unique arrangement of cysteine and histidine residues. The study suggests that the repetitive structure of TFIIIA may have evolved through gene duplication or gene conversion, allowing it to perform multiple functions in the 5S gene control system. The protein's structure and function are important for understanding the regulation of 5S RNA transcription in Xenopus oocytes.