A novel illegitimate recombination event: precise excision and reintegration with the Mu gem mutant prophage

Area of Science:

• Bacteriophage genetics
• Molecular biology
• Microbial genetics

Background:

• Bacteriophage Mu typically integrates randomly into the Escherichia coli chromosome.
• Unlike transposable elements, wild-type Mu does not exhibit precise prophage excision, which restores the original DNA sequence.
• Precise excision has been previously reported only for specific defective Mu mutants.

Purpose of the Study:

• To investigate the precise excision capabilities of the mutant bacteriophage Mu gem2ts.
• To understand the conditions and genetic dependencies of Mu gem2ts excision.
• To elucidate the mechanism behind the paradox of limited precise Mu excision.

Main Methods:

• Utilizing the mutant prophage Mu gem2ts in Escherichia coli.
• Selecting for precise excision events at specific loci (malT, lac, thyA).
• Assessing excision under permissive conditions, in immune lysogens, and evaluating RecA and Mu transposase independence.

Main Results:

• The mutant prophage Mu gem2ts demonstrates precise excision from at least three distinct loci (malT, lac, thyA).
• Excision occurs under permissive phage development conditions and in immune lysogens, independent of RecA and Mu transposase.
• Mu gem2ts excision is consistently followed by reintegration of the prophage elsewhere in the E. coli chromosome, notably at the 94-min locus for Mal+ revertants.

Conclusions:

• Mu gem2ts exhibits precise excision, challenging the general understanding of Mu prophage behavior.
• The mechanism of Mu gem2ts excision provides insights into the long-standing question of why precise Mu excision is rare.
• This finding contributes to understanding bacteriophage integration and excision dynamics in bacterial genomes.