Integration of bacteriophages lambda and phi 80 in wild-type Escherichia coli at secondary attachment sites. I. Formation of secondary lysogens.

The family of lambdoid phages displays a varying specificity of integration into the host chromosome. The lambda phage DNA failed to get inserted at the secondary site(s) of the gal operon (frequency less than 2.6 X 10(-8) in the presence of the primary (normal) att site. By contrast, phi 80 and the lambda att80 hybrid (lambda X phi 80) became integrated into wild-type Escherichia coli at at least two secondary att sites of the btuB locus, and the latter near purE and purC as well (frequency 2 X 10(-3)-10(-4). The integration of phi 80 and lambda att80 into btuB occurred with about the same frequency as in cells in which the normal insertion site had been deleted (0.7-4.0 X 10(-6). An analysis of the secondary lysogens with the prophage in btuB showed them to be polylysogens; the additional prophage(s) was found at the primary att site. We also failed to observe the integration into other loci of phi 80 and lambda att80 with the formation of secondary monolysogens (frequency less than 0.0035 at MOI = 10(-3) or 10). It is presumed that these prophages become integrated at secondary att sites only if the primary site is occupied.