In vitro construction of different oligomeric forms of lambdadv DNA and studies of their transforming activities

Plasmid lambdadv1, which is in a dimeric form, was converted to a linear monomer duplex by the action of EcoRI restriction endonuclease that incises at a unique site in this plasmid genome. The resulting products were then joined by Escherichia coli DNA ligase to produce molecules with various oligomeric forms, and from these monomeric, dimeric, or trimeric circular molecules were purified. By transformation of cells with these DNAs, clones were obtained that carried lambdadv1 in a monomeric or dimeric form. The former type of clones have not been generated in vivo, except for one in a different host strain, and carriers of timeric or tetrameric lambdadv1's have not been obtained so far. It was observed that a considerable fraction of these oligomeric circular DNAs were converted to lower oligomers (e.g., from trimer to dimer) during transformation. The characteristics of the monomeric lambdadv1 carriers obtained were compared with those of dimeric lambdadv1 carriers. The stabilities of the plasmids of the two forms were the same. However, the monomeric plasmid carriers were less tolerant to lambdavir phage infection and perpetuated about 30% less plasmid genomes in monomer units. Furthermore, dimeric plasmid carriers appeared spontaneously and accumulated in cultures of the monomeric lambdadv1 carriers.