The content of ColE 1-related plasmids increased about 4-6-fold after a temperature-shift from 30 to 42 degrees C (45 degrees C) if the rom-region of the plasmids was deleted. The copy number of rom(+)-plasmids did not change after the temperature shift. All rom(-)-plasmids tested in this study showed this plasmid amplification. The Rom-protein is capable of inhibiting plasmid replication by stabilization the initial reversible stage of the association of RNA I with the primer precursor RNA II. We suggest that the temperature-dependent enhancement of the copy number of rom(-)-plasmids is due to a destabilization of this initial phase of the RNA I-preprimer interaction at high temperatures which is suppressed by the Rom-protein in cells with rom(+)-plasmids.
