DNA-binding properties of a Lac repressor mutant incapable of forming tetramers

Brenowitz, M. ; Mandal, N. ; Pickar, A. ; Jamison, E. ; Adhya, S. (1991) DNA-binding properties of a Lac repressor mutant incapable of forming tetramers Journal of Biological Chemistry, 266 . pp. 1281-1288. ISSN 0021-9258

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Official URL: http://www.jbc.org/content/266/2/1281.short


The interaction of proteins bound to sites widely separated on the genome is a recurrent motif in both prokaryotic and eukaryotic regulatory systems. Lac repressor mediates the formation of "DNA loops" by the simultaneous interaction of a single protein tetramer with two DNA-binding sites. The DNA-binding properties of a Lac repressor mutant (LacIadi) deficient in the association of protein dimers to tetramers was investigated. The results of quantitative footprint and gel mobility-shift titrations suggest that the wild-type Lac repressor (LacI+) binds cooperatively to two operator sites separated by 11 helical turns on a linear DNA restriction fragment by the formation of a "looped complex." LacIadi binds to this two-site operator non-cooperatively and without formation of a looped complex. These results demonstrate that the dimer-tetramer association of LacI+ is directly responsible for its cooperative binding and its ability to mediate formation of a looped complex. The Iadi mutation disrupts the monomer-dimer as well as eliminating the dimer-tetramer association equilibria while the DNA binding affinity of LacIadi to a single site is unchanged relative to the wild-type protein. These results suggest that DNA binding and dimer-tetramer association are functionally unlinked. The similarity of the DNA-binding properties of LacIadi and Gal repressor, a protein believed to function by mediating the formation of a looped complex, are discussed.

Item Type:Article
Source:Copyright of this article belongs to The American Society for Biochemistry and Molecular Biology.
ID Code:74584
Deposited On:17 Dec 2011 10:03
Last Modified:17 Dec 2011 10:03

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