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Volume 272, Number 35, Issue of August 29, 1997 pp. 22092-22096
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Comparison of the DNA Association Kinetics of the Lac Repressor Tetramer, Its Dimeric Mutant LacI^adi, and the Native Dimeric Gal Repressor

(Received for publication, January 23, 1997, and in revised form, May 22, 1997)

From the Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York 10461

The rates of association of the tetrameric Lac repressor (LacI), dimeric LacI^adi (a deletion mutant of LacI), and the native dimeric Gal repressor (GalR) to DNA restriction fragments containing a single specific site were investigated using a quench-flow DNase I "footprinting" technique. The dimeric proteins, LacI^adi and GalR, and tetrameric LacI possess one and two DNA binding sites, respectively. The nanomolar protein concentrations used in these studies ensured that the state of oligomerization of each protein was predominantly either dimeric or tetrameric, respectively. The bimolecular association rate constants (k_a) determined for the LacI tetramer exceed those of the dimeric proteins. The values of k_a obtained for LacI, LacI^adi, and GalR display different dependences on [KCl]. For LacI^adi and GalR, they diminish as [KCl] increases from 25 mM to 200 mM, approaching rates predicted for three-dimensional diffusion. In contrast, the k_a values determined for the tetrameric LacI remain constant up to 300 mM [KCl], the highest salt concentration that could be investigated by quench-flow footprinting. The enhanced rate of association of the tetramer relative to the dimeric proteins can be modeled by enhanced "sliding" (Berg, O. G., Winter, R. B., and von Hippel, P. H. (1981) Biochemistry 20, 6929-6948) of the LacI tetramer relative to the LacI^adi dimer or a combination of enhanced sliding and the superimposition of "direct transfer" mediated by the bidentate DNA interactions of the tetramer.

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