Integration of bacteriophages λ and ⊘80 in wild-type Escherichia coli at secondary attachment sites

Summary

The frequency of occurrence and the genetic structure of polylysogens were studied for phages λ, ⊘80 and λatt80. In the case of λ, frequency of polylysogenization is high (0.20 to 0.41) with a tandem integration of prophages at the primary att site (attλ). With ⊘80 and λatt80, this frequency is about 10 times lower, and usually one prophage becomes integrated at the primary att site (att80-I) while another (sometimes two others) integrates at one of the secondary sites. At least four secondary att80 sites have been found in wild-type Escherichia coli ¹, two of which (near the his and tolC loci) are preferred. The frequency of secondary integration of ⊘80 and λatt80 does not differ significantly in the wild-type host and in that deleted for the primary att site (0.041 and 0.045, respectively, among surviving cells at an MOI of 10).

Homoimmune superinfection has revealed a constitutive cI-independent expression of the ⊘80 int gene in the prophage state. The only ⊘80 tandem detected proved to be unstable. With the ⊘80int ^- mutant, we observed stabilization of ⊘80 tandems; as a consequence, their frequency of occurrence during coinfection with ⊘80int ⁺ was up to the λ level and no nontandem insertions were found. A model is proposed for the ⊘80 and λatt80 nontandem integration.