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

• Authors: Geraldine Fulcrand, Florida International University; Samantha Dages, Florida International University; Xiaoduo Zhi, Florida International University; Prem Chapagain, Florida International University; Bernard S. Gerstman, Florida International University; David Dunlap, Emory University; Fenfei Leng, Florida International University
• Journal Title: Scientific Reports
• Volume: Volume 6
• Publisher: Nature Publishing Group | 2016-01-14, Pages 19243-19243
• Type of Work: Article | Final Publisher PDF
• Publisher DOI: http://doi.org/10.1038/srep19243
• Subjects: Biology, Genetics; Biophysics, General; Chemistry, Biochemistry
• Keywords: Bacterial genetics; Transcriptional regulatory elements

Abstract

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.