ADAPTATION OF ESCHERICHIA-COLI GROWTH-RATES TO THE PRESENCE OF PBR322

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 (mu(max)) determined. The acquisition of pBR322 by the host resulted in a decreased mu(max). Following repeated subculturing of the plasmid-containing strain on selective medium, restoration in mu(max) 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 plasmids with and without culture histories, indicated that the site of the adaptive mutation was located on the host chromosome rather than on the plasmid.