Plasmid addiction system in high-copy number plasmids
According to the review by Kroll J, Klinter S, Schneider C, Voß I, Steinbüchel A. Plasmid addiction systems: perspectives and applications in biotechnology. Microbial biotechnology. 2010;3(6):634-657. doi:10.1111/j.1751-7915.2010.00170.x., there are high-copy-number Plasmid Addiction Systems (PAS), "specifically the addiction system depending on plasmid‐borne ORT utilizing the lac repressor."


  The lac repressor protein (LacI) binds to the lac operator (lacO) and therefore represses the expression of the kanamycin resistance gene. When IPTG is available as an inducer, LacI is titrated by lacO, and the E. coli cells were able to grow in media containing kanamycin due to expression of the corresponding resistance gene. The same result was obtained when high‐copy‐number plasmids harbouring lacO were used for ORT. In this case the plasmid was stably maintained over 72 generations (Williams et al., 1998).


ORT = operator repressor titration 

ORT is one of the three main groups of PAS.