DNA supercoiling, a critical signal regulating the basal expression of the lac operon in Escherichia coli

Sci Rep. 2016 Jan 14;6:19243. doi: 10.1038/srep19243.


Escherichia coli lac repressor (LacI) is a paradigmatic transcriptional factor that controls the expression of lacZYA in the lac operon. This tetrameric protein specifically binds to the O1, O2 and O3 operators of the lac operon and forms a DNA loop to repress transcription from the adjacent lac promoter. In this article, we demonstrate that upon binding to the O1 and O2 operators at their native positions LacI constrains three (-) supercoils within the 401-bp DNA loop of the lac promoter and forms a topological barrier. The stability of LacI-mediated DNA topological barriers is directly proportional to its DNA binding affinity. However, we find that DNA supercoiling modulates the basal expression from the lac operon in E. coli. Our results are consistent with the hypothesis that LacI functions as a topological barrier to constrain free, unconstrained (-) supercoils within the 401-bp DNA loop of the lac promoter. These constrained (-) supercoils enhance LacI's DNA-binding affinity and thereby the repression of the promoter. Thus, LacI binding is superhelically modulated to control the expression of lacZYA in the lac operon under varying growth conditions.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • DNA, Bacterial* / ultrastructure
  • DNA, Superhelical* / ultrastructure
  • Escherichia coli / genetics*
  • Escherichia coli / metabolism
  • Gene Expression Regulation, Bacterial*
  • Lac Operon*
  • Lac Repressors / chemistry
  • Lac Repressors / metabolism
  • Promoter Regions, Genetic
  • Protein Binding
  • Protein Multimerization


  • DNA, Bacterial
  • DNA, Superhelical
  • Lac Repressors