complex between gyrase and DNA and associates with DNA gyrase to produce a complex that is impaired in supercoiling activity (61). These combined effects are lethal for E. coli.

A variety of different toxin-antitoxin systems are widely disseminated on bacterial plasmids, although the intracellular targets for the toxin components of these systems probably differ. Toxin-antitoxin homologs have also been identified on many bacte­rial chromosomes, where they might function as bacterial programmed cell death sys­tems during periods of nutritional and other stresses (57).

Large, low-copy-number plasmids often utilize partition, recombination, and toxin-antitoxin systems to promote segregational stability. The segregational maintenance of these plasmids is achieved through the activity of all three mechanisms.