DNA replication regulated by the priming promoter.

ColE1 derivative plasmids were constructed in which the natural promoter that primes replication or, in addition, the region coding for the RNA I control element had been deleted. In all of these molecules priming of the origin was effected by read-through transcription from constitutive or inducible (lacUV5) promoters inserted farther upstream. In the latter case, regulation of lac repressor activity with IPTG resulted in controlled plasmid levels in vivo. These results indicated that, at least in the absence of other control elements, regulation of the priming promoter was sufficient to control DNA replication and determine plasmid copy number.