If a plasmid lacks the A. gene el, which produces the lysis repressor, it multiplies like a phage and produces plaques on a bacterial lawn. But if el gene is present, the plasmid replicates like a plasmid.
Further, a plasmid may contain a mutant cI gene, which produces a temperature sensitive CI protein (inactive at higher temperatures); such vectors replicate as plasmids at lower temperatures, but behave like phage at higher temperatures. This feature is quite useful in some experiments.
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