Letter: functional form of RNA synthesis termination factor rho

The ability of rho factor to effect in vitro termination of RNA synthesis requires the association of rho monomers into an oligomer consisting of at least four subunits. It was found that (1) rho factor activity has a sigmoidal dependence upon concentration, and (2) rho factor's sedimentation coefficient decreases with decreasing concentration, proceeding through 8.5 S (tetramer), 6.2 S (dimer) and finally 4.3 S (monomer) forms as the concentration approaches the apparent equilibrium binding constant. Rho factor may function at specific sites in the DNA template through the co-operative binding of subunits into an oligomer which surrounds the DNA helix.