GalR mutants defective in repressosome formation

Area of Science:

• Molecular Biology
• Genetics
• Protein Structure and Function

Background:

• Transcription repression of the galactose operon in E. coli involves the GalR repressor, histone-like protein HU, and supercoiled DNA.
• These components form a DNA loop structure known as a repressosome through protein-protein interactions.

Purpose of the Study:

• To investigate the structural basis of repressosome formation by analyzing GalR mutants with defects in DNA looping.
• To identify specific amino acid residues in GalR involved in protein-protein interactions within the repressosome.

Main Methods:

• Screening and isolation of galR mutants with impaired DNA loop formation but intact operator-binding activity.
• Purification and in vitro characterization of mutant GalR proteins.
• Circular dichroism spectroscopy to assess protein secondary structure.
• Homology modeling of GalR based on crystal structures of related proteins (PurR, LacI).

Main Results:

• Single amino acid substitutions in GalR were found to disrupt DNA looping while preserving operator binding.
• Mutations were localized to the surface-exposed carboxy-terminal subdomain of GalR, suggesting roles in protein interactions.
• Structural modeling and comparison with LacI and PurR identified potential GalR-GalR interaction sites.
• Mutant phenotypes were consistent in the presence of both HU and the unrelated eukaryotic protein HMG-17.

Conclusions:

• The identified mutations pinpoint critical GalR-GalR interaction sites involved in repressosome-mediated DNA looping.
• These findings provide structural insights into the mechanism of transcription repression by the GalR protein.