Phasmids: hybrids between Co1E1 plasmids and E. coli bacteriophage lambda

Plasmids carrying cloned λ att sites may be integrated into the bacteriophage genome by the site-specific recombination mechanism of λ. The cross, referred to as "lifting" the plasmid, requires mixed infection of an Escherichia coli strain carrying the plasmid with two appropriately constructed "lifting" λ phages. One phage donates a short left arm and the other donates a short right arm. These two short arms are of insufficient length to produce a viable phage genome and yield no recombinants when crossed on standard bacteria. However, viable recombinants are obtained when the genome length is extended by integration of one or more plasmids. We call these recombinants phasmids. They contain multiple att sites introduced at the ends of the integrated plasmids, and in the presence of integrase, recombination between these att sites can be exploited to effect release of the plasmid components. These novel genetic elements can be used in a variety of ways as vectors in genetic manipulation experiments. Sequences cloned in phasmids may be studied as a component of either a plasmid and or of a phage, and easily interconverted between the two states.