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QUARTERLY Review Mechanisms of plasmid stable maintenance with special focus on plasmid addiction systems �

By Urszula Zielenkiewicz


The stable inheritance of bacterial plasmids is achieved by a number of different mechanisms. Among them are resolution of plasmid oligomers into monomers, active plasmid partitioning into dividing cells and selective killing of plasmid-free segregants. A special focus is given to the last mechanism. It involves a stable toxin and an unstable antidote. The antidotes neutralize their cognate toxins or prevent their synthesis. The different decay rates of the toxins and the antidotes underlie molecular mechanisms of toxin activation in plasmid-free cells. By eliminating of plasmid-free cells from the population of plasmid-bearing ones the toxin-antidote couples therefore act as plasmid addiction systems. Plasmids are separate, autonomous genetic elements present in a cell independently of chromosomes. Most plasmids are small: from several to 100 kb, but sometimes they are so large that using the size criteria their distinction from the chromosome is difficult (e.g. in Vibrio cholerae, Yamaichi et al., 1999; in Rhizobium meliloti, Honeycutt et al., 1993). Different plasmids can constitute even up to 50 % of bacterial DNA (e.g. in Borrelia burgdorferi, Fraser et al., 1997; Bacillus cereus, Carlson & Kolstø, 1994). It is commonly accepted that plasmid genes do not encode information indispensable for the functioning of the host cell. However, plasmids specify numerous features advantageous for the host in specific environments, such as resistance to harmful agents, ability to degrade rare compounds, pathogenicity, toxin production, nitrogen assimilation etc

Topics: Key words, plasmid addiction, post-segregational killing, partition, multimer resolution
Year: 2001
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