High-molecular-weight linear multimer formation by single-stranded DNA plasmids in Escherichia coli.

We inserted foreign DNA segments into plasmids which replicate by a rolling-circle mechanism in Escherichia coli and observed the appearance of high-molecular-weight plasmid multimers (HMW). This phenomenon, which occurs more frequently with GC-rich segments, depends on the mode of replication of the plasmid and on host homologous recombination functions. We found that (i) HMW are formed upon insertion of a foreign DNA segment into a single-stranded DNA plasmid, whereas the same DNA insert has no such effect on a theta replicon, and (ii) HMW are not present in a recA mutant strain but are found in a lexA (Ind-) mutant. Enzymatic studies allowed us to define the HMW structure as linear double-stranded tandem head-to-tail plasmid repeats. Use of heteroplasmid strains showed that HMW production by one plasmid does not affect another resident plasmid, indicating that no host functions are phenotypically inactivated. This distinguishes our system from the HMW observed with various replicons in the absence of RecBCD enzyme activity. We propose that the role of the foreign insert is to protect the DNA from RecBCD exonuclease attack.