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 (LacI(adi)) 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.' LacI(adi) 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 I(adi) mutation disrupts the monomer-dimer as well as eliminating the dimer-tetramer association equilibria while the DNA binding affinity of LacI(adi) 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 LacI(adi) and Gal repressor, a protein believed to function by mediating the formation of a looped complex, are discussed.
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of Biological Chemistry|
|Publication status||Published - Feb 19 1991|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology