Adaptation of Escherichia coli growth rates to the presence of pBR322

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Abstract
Changes in the growth rate of Escherichia coli K12 J62-1 in response to the presence of plasmid pBR322 have been investigated. Plasmid-free and plasmid-containing strains were grown in batch culture and their maximum specific growth rate (μmax) determined. The acquisition of pBR322 by the host resulted in a decreased μmax. Following repeated subculturing of the plasmid-containing strain on selective medium, restoration in μmax was observed. The copy number and structure of the plasmid were not significantly altered during the experiment Growth rate measurements for a series of strains constructed using a combination of host cells and plas-mids with and without culture histories, indicated that the site of the adaptive mutation was located on the host chromosome rather than on the plasmid.