FORMATION OF LINEAR PLASMID MULTIMERS PROMOTED BY THE PHAGE-LAMBDA RED-SYSTEM IN LON MUTANTS OF ESCHERICHIA-COLI

We report here the formation of plasmid linear multimers promoted by t he Red-system of phage lambda using a multicopy plasmid comprised of l ambda redalpha and redbeta genes, under the control of the lambda cI85 7 repressor. Our observations have revealed that the multimerization o f plasmid DNA is dependent on the redbeta and recA genes, suggesting a concerted role for these functions in the formation of plasmid multim ers. The formation of multimers occurred in a recBCD+ sbcB+ xthA+ lon genetic background at a higher frequency than in the isogenic lon+ hos t cells. The multimers comprised tandem repeats of monomer plasmid DNA . Treatment of purified plasmid DNA with exonuclease III revealed the presence of free double-chain ends in the molecules. Determination of the size of multimeric DNA, by pulse field gel electrophoresis, reveal ed that the bulk of the DNA was in the range 50-240 kb, representing a pproximately 5-24 unit lengths of monomeric plasmid DNA. We provide a conceptual framework for Red-system-promoted formation and enhanced ac cumulation of plasmid linear multimers in lon mutants of E. coli.