Persistence of pBR322-related plasmids inEscherichia coligrown in chemostat cultures

The stability of plasmid pBR322 and a number of close derivatives was examined by continuous culture of Escherichia coli. Cultures were subjected to either glucose, phosphate or magnesium limitation in non-selective medium at a dilution rate of 0.1/h. Under these conditions pBR322 was eventually lost from the population, but only after a distinct lag period. The closely related plasmids pBR325 and especially pBR327 and pBR328, but not pAT153, were lost more rapidly. Three cosmids pHC79, pSJ55 and pJB8 were generally found to be less stable than the pBR322-type plasmids from which they were derived. Chimaeric plasmids containing DNA from yeast and from a thermophilic bacillus were also unstable in E. coli.