Preparation of plasmids from lambdoid phages and studies on their incompatibilities

Abstract

Various lambdoid phages, including those carrying the immunity region of 434, 21, and φ80, were found to give rise to fragments of DNA that can be perpetuated in the plasmid state. The plasmids, imm434dv, imm21dv, and φ80dv, were similar to the already known plasmid λdv in size, genetic constitution, oligomeric state, copy number, and stability. Cells carrying two kinds of dv plasmids were constructed by transformation. It was shown that a pair of plasmids is compatible in a cell if they originate from phages differing in immunity region. On the other hand, a pair of plasmids is incompatible if they are derived from phages carrying the same immunity region. These observations were taken to imply that the incompatibility of a pair of plasmids is determined by the “immunity region” of the plasmid genome that contains an autorepressor gene and a promoter-operator. The region that carries initiator genes and a site for initiation of plasmid replication is not primarily important for determination of the incompatibility. Plasmids λdv and imm434dv, which are very closely related to each other, behaved in an intermediate fashion.