Second-element turn-on of gene expression in an IS1 insertion mutant

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• Insertion sequences (IS) are mobile genetic elements that can disrupt gene function.
• Polar mutations, like galOP-306::IS1 in Escherichia coli K12, can silence gene expression.
• Understanding gene reactivation mechanisms is crucial for genetic research.

Purpose of the Study:

• To investigate the reactivation of silenced genes caused by insertion mutations.
• To systematically analyze the genetic alterations leading to the restoration of gal operon expression.

Main Methods:

• Studied Gal+ revertants of the polar mutant galOP-306::IS1 in Escherichia coli K12.
• Utilized selective conditions to exclude precise IS1 excision as a reversion mechanism.
• Analyzed the structural alterations within the IS1 element in revertant strains.

Main Results:

• Reversion to the Gal+ phenotype occurred at a frequency of approximately 10(-7) per cell per generation.
• Most reactivation events (all but one of ~200 studied) involved alterations in IS1 structure.
• These alterations included insertions of IS2, insertion of a new element IS150, and a small deletion within IS1.

Conclusions:

• Alterations in the IS1 insertion sequence are the primary mechanism for reactivating the silenced gal operon in this system.
• The intervention of a second mobile element (IS2 or IS150) is the predominant mode of IS1 alteration leading to gene reactivation.
• This study identifies IS150 as a novel mobile element involved in gene regulation.