Kinetics of protein-nucleic acid interactions: use of salt effects to probe mechanisms of interaction.

The kinetics of protein-nucleic acid interactions are discussed with particular emphasis on the effects of salt concentration and valence on the observed rate constants. A general review is given of the use of experimentally determined salt dependences of observed kinetic parameters as a tool to probe the mechanism of interaction. Quantitative analysis of these salt dependences, through the application of polyelectrolyte theory, can be used to distinguish reactions which occur in a single step from those reactions which involve distinct intermediates. For those rate constants which display a large salt dependence, in either the association or dissociation reaction, this is due to the high concentration of counterions (e.g., Na+) in the vicinity of the nucleic acid which are subsequently released (or bound in the case of dissociation) at some point before the rate limiting step of the reaction. A general discussion of other features which affect protein-nucleic acid kinetics, such as nucleic acid length and the ratio of nonspecific to specific DNA binding sites (in the case of sequence specific binding proteins), is also given. The available data on the nucleic acid binding kinetics of small ligands (ions, dyes, oligopeptides), nonspecific binding proteins (T4 gene 32 protein, fd gene 5 and Escherichia coli SSB), and sequence specific binding proteins (lac repressor, RNA polymerase, Eco RI restriction endonuclease) are discussed with emphasis on the interpretation of the experimentally determined salt dependences.