Recently we have developed a general theory to interpret ion concentration effects on the equilibrium and kinetics of protein-nucleic acid interactions. We are able to calculate the number of ion pairs involved in an interaction, and the contribution of these ion pairs to the free energy of complex formation. Furthermore, we are able to decide between various possible mechanisms for an interaction on the basis of the ion concentration dependences of the rate constants. We have previously applied this approach to the extensive literature data for the lac repressor-operator interactions. Here we propose experiments to determine the effects of ion concentration on the equilibrium and kinetic constants for the interactions of E. coli RNA polymerase (both holoenzyme and core polymerase) with double-and single-stranded DNA, and for the specific interaction of holoenzyme with promoter sequences. Our approach, using ions as probes of the thermodynamics and mechanism of the polymerase-DNA interactions, is a novel and powerful one. We will be able to determine the electrostatic and non-electrostatic components of the interaction free energy, characterize the size and nature of the polymerase binding site(s) for DNA, characterize the putative closed and open enzyme-promoter complexes, develop a mechanism for formation of these complexes, and obtain information on the role of initiation factor sigma in the regulation of gene expression. In related work we will study solvent and monovalent and divalent ion effects on enzyme aggregation and on the binding of enzyme or enzyme aggregates to DNA.