ATP and other nucleotides stabilize the Rho-mRNA complex

Transcription termination factor Rho from Escherichia coli is a protein that consists of a single 47 kDa protomeric unit that can form a hexameric structure. To determine whether active hexamers can form on an RNA by assembly of subunits, we measured the dependence of complex formation on the concentration of Rho protein in the presence and absence of various nucleotides and related the binding properties to association states determined from sedimentation properties. The results show that the presence of adenine nucleotides converts RNA binding from a multimeric process to a largely monomeric process and that the change correlates with the stabilization of multimers of Rho by the nucleotides. The experimental evidence also indicates that the hexameric form of Rho is stabilized slightly by binding to a transcript but that the protein on RNA is in equilibrium with nonhexameric forms. These results suggest that a Rho hexamer can form on a transcript by addition of subunits to a partial assembly, which means that the complex can consist of six subunits surrounding an RNA transcript as proposed in recent models for Rho action.