CHROMOSOMAL INTEGRATION OF PHAGE-LAMBDA BY MEANS OF A DNA INSERTION ELEMENT

Phage λcam112, which contains the chloramphenicol resistance transposon Tn9 and has a deletion of attP and the int gene, will lysogenize Escherichia coli K-12. Prophage integration occurs at different chromosomal sites, including lacY and malB, but not at attB. All λcam112 prophages are excised from the chromosome after induction but with various efficiencies for different locations. Heteroduplex analysis of λplacZ transducing phages isolated from a lacY::λcam112 prophage reveals an insertion sequence 1 (IS1) element at the joint of viral and chromosomal DNA. Two lines of evidence indicate that λcam112 encodes an excision activity that recognizes the IS1 element:(i) prophage derepression increases the frequency of excision from lacY to yield lac+ revertants, and (ii) λcam112 infection increases reversion of a galT::IS1 mutation about 50-fold. Our results indicate that the IS1 termini of Tn9 can replace attP as a site for λ insertion in the bacterial chromosome and that excision events are catalyzed by an IS1-encoded protein under λ repressor and N gene control.