impairs further elongation of this transcript, thereby reducing the frequency of RNA-DNA duplex formation and initiation of rep­lication. The RNAI-RNAII interaction is counterbalanced by the shorter half-life of RNAI compared to RNAII. The ColE1-encoded Rom protein (also known as Rop) increases the frequency of RNAI-RNAII interactions. The gene for Rom is deleted in many ColE1-based plasmid vectors, resulting in increased copy numbers compared to ColE1 itself. Perturbations of ColE1 plasmid copy number are rapidly mirrored by changes in RNAI concentration, resulting in the enhancement or suppression of repli­cation and the maintenance of ColE1 copy number within a narrow window.