Copy Mutants of the Plasmid R 1 as a Tool in Studies of Control of Plasmid Replication
Replication of bacterial chromosomes and plasmids is carefully regulated; their cellular concentration is kept constant in an exponentially growing population of bacteria /4/, /13/. Replication is controlled at the level of initiation /38/. The initiation mass (cell mass per origin) of the Escherichia coli chromosome is constant at least at growth rates above one generation per hour suggesting that initiation mass is the parameter used to control replication /10/, /36/, /38/. This is a reasonable idea since it requires measurement of a concentration, a principle often used in biochemical processes. Two main models have been proposed to explain the coupling between replication and growth, the one suggesting positive /19/, the other negative control /37/, /39/. Many experiments have been performed in order to distinguish between these two models, but almost no definitive answers have so far been obtained. Control of DNA replication differs from the control of synthesis of other cellular macromolecules in that the number of molecules that are synthesized per cell (and per cell generation) is limited; in some cases this number is only one. This particularly applies to the chromosome but also to many plasmids. on the other hand, it is absolutely necessary that the process is carefully controlled, otherwise cells lacking the DNA molecule could become abundant.
KeywordsPlasmid Copy Plasmid Copy Number Plasmid Replication Chromosome Replication Newborn Cell
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