Effect of lac repressor oligomerization on regulatory outcome

Regulatory outcome in a bacterial operon depends on the interactions of all the components which influence mRNA production. Levels of mRNA can be altered profoundly by both negative and positive regulatory elements which modulate initiation of transcription. The occupancy of regulatory sites on the DNA by repressors and activators is determined not only by the affinity of these proteins for their cognate site(s) but also by the oligomeric state of the regulatory protein. The lac operon in Escherichia coli provides an excellent prototypic example of the influence of protein assembly on the transcriptional status of the associated structural genes. DNA loop formation is essential for maximal repression of the lac operon and is contingent upon the presence of multiple operator sites in the DNA and the ability of the repressor to self-associate to form a bidentate tetramer. The stability of this looped complex is enhanced significantly by DNA supercoiling. Tetramer assembly from dimers apparently occurs via interactions of a 'leucine zipper' motif in the C-terminal domain of the protein, and the tetramer is essential to formation of looped complexes. Furthermore, analysis of the DNA-binding characteristics of dimeric mutants has established that the monomer-dimer association and dimer-DNA binding (monomer does not bind to DNA) are coupled equilibria. Thus, dimer assembly is essential for generating a DNA-binding unit, and tetramer assembly is required for formation of the stable looped DNA structure that maximally represses mRNA synthesis. Protein-protein interactions therefore play a pivotal role in the regulatory activities of the lac repressor and must be considered when analysing the activities of any oligomeric DNA-binding protein.