Escherichia coli relA strains as hosts for amplification of pBR322 plasmid DNA

Abstract We have proposed that guanosine tetraphosphate produced in Escherichia coli cells subjected to an isoleucine limitation inhibits pBR322 DNA replication [1]. In E. coli relA which cannot synthesize guanosine tetraphosphate (ppGpp) upon amino acid limitation pBR322 DNA is amplified after arginine starvation. The yield of plasmid DNA amplified either by chloramphenicol (Cm) or by arginine limitation is compared. The plasmid yield per cell is equal in amino acid-starved cells and in cells treated with Cm. To increase the plasmid content per ml of cell suspension the growth medium was supplemented with increasing amounts of nutrients. Plasmid DNA can be isolated in large quantities by this procedure. This simple method can be used for the enrichment of pBR325 DNA which cannot be amplified by Cm treatment. Our results indicate that E. coli relA strains might be suitable hosts for the amplification of pBR322 and related plasmids in E. coli .