Amplification of different ColE1 plasmids in an Escherichia coli relA strain

Amino acid starved cells of an E. coli relA strain accumulate a large amount of pBR322 plasmid DNA. In this study ColE1 related plasmids of different copy number and size including a high copy number plasmid mutant of pBR322 were amplified in a relA strain of E. coli K-12 under amino acid limitation in order to determine the upper plasmid level in amino acid starved cells. In all cases we measured a 4 to 6 fold increase of the plasmid copy number in comparison to log-phase cells independent of the size, the number of origins per plasmid molecule or the copy number in log-phase cells. The plasmid copy number in amino acid starved cells varies from about 200 (pBR322-dimer) to about 2000 (high copy number plasmid pERIII-BPL4, see Boros et al. 1986). Rop+ and rop- plasmids show the same amplification rate under the used conditions.