Escherichia coli σ70 and NusA proteins

In this paper we examine the physical properties and potential for self-association of the Escherichia coli transcription factors, σ 70 and NusA. We show, by a combination of chemical crosslinking, equilibrium and velocity sedimentation, quasi-elastic light scattering, and small-angle X-ray scattering that NusA exists as a monomer at KCl concentrations between 0.01 and 1.5 m , and that σ 70 exists as a monomer at KCl concentrations between 0.1 and 1.5 m . The shape and hydration characteristics of each of these monomeric proteins are also examined. The results serve as background for the companion paper in which a thermodynamic analysis is made of the interactions of these transcription factors with E. coli core RNA polymerase in solution and as a component of the functional transcription complex.