Bacteriophage lambda derivatives carrying two copies of the cohesive end site

A spontaneously arising tandem duplication derivative of bacteriophage lambda has been isolated, which carries two copies of the site where the cohesive ends are formed (designated cos). Its structure has been determined by electron microscopy of DNA heteroduplexes. These heteroduplexes reveal that the duplication is usually, but not always, carried on the left end of the chromosome. A second duplication phage having two copies of cos, constructed by Feiss & Campbell (1974), has also been studied by electron microscopy and is found to have a similar property. Unlike most tandem duplication derivatives of phage λ, the mutant studied here is not stable during growth in the absence of generalized recombination, but segregates both the triplication and the parental phage. This verifies that both cos sites are functional. The triplication does not arise as a result of end-to-end aggregation of phage chromosomes or site-specific recombination catalyzed by the chromosome maturation system at cos. It must therefore result from the cutting of mature ι chromosomes from concatemeric replication intermediates. The pattern of cutting observed shows that the λ cohesive ends are not created by a free nuclease acting on unpackaged DNA. The cutting appears to be influenced by the amount of DNA previously packaged into a phage head. A model for λ packaging is presented which explains the results. The duplication phage of Feiss & Campbell (1974) carries a novel addition containing self-complementary sequences.