7S particles containing 5S RNA and the transcription regulatory protein factor A have been purified to near homogeneity from Xenopus laevis oocytes. The binding of the transcription factor to the Xenopus borealis somatic 5S RNA gene has been monitored by quantitating the DNase I protection patterns of the protein-DNA interaction. Under stoichiometric binding conditions--i.e., when all added DNA binds to the factor--two protein molecules are required to saturate the 5S RNA gene. Under equilibrium binding conditions, titration of the 5S RNA gene with factor A results in a sigmoidal binding isotherm suggesting a cooperative interaction; half-saturation of binding is observed at a free factor A concentration of 1 nM. Cooperative binding between factor A and the 5S RNA gene may contribute to the stability of the transcription apparatus and its maintenance during cell division.