Differential replication of plasmids during stringent and relaxed response of Escherichia coli.

Stringent control of DNA replication has been demonstrated for a few replicons like oriC, pBR322, and plasmids derived from coliphage lambda. In this study we investigated the replication of other plasmids harboring a well defined origin (orip15A, oripSC101, and oriRK2 = oriV) in amino-acid-starved stringent and relaxed strains of Escherichia coli. We found differential replication of plasmids during stringent and relaxed response. Inhibition of DNA synthesis or amplification of plasmid DNA in amino acid-starved relA+ and relA- cells depends on the kind of replicon and, surprisingly, on the nature of deprived amino acid. We conclude that there are no general rules for stringent control of DNA replication and each replicon must be considered separately. There are, however, possible explanations for the differences shown between replicons in their response to stringent and relaxed conditions.