STRINGENT CONTROL OF REPLICATION OF PLASMIDS DERIVED FROM COLIPHAGE LAMBDA

The first events of lambda-plasmid replication in vivo, which probably regulate this process, are the transcriptional activation of the origin of replication by RNA polymerase and the binding of the initiator protein, lambda-O, to this nucleotide sequence. The lambda-O protein is known for its rapid proteolytic degradation; hence amino acid starvation of Escherichia coli should result in inhibition of lambda plasmid replication caused by inhibition of protein synthesis. However, contrary to this prediction, we found that lambda-plasmid replication, as measured by the increase in plasmid content per bacterial mass, proceeds for hours in an amino acid-starved, relaxed mutant, whereas it is inhibited in its wild-type stringent partner. lambda-plasmid replication in amino acid-starved, relaxed cells reveals absolute lambda-O dependence and is not inhibited by chloramphenicol at 200 mu-g/ml. This process also occurs in wild-type cells treated with chloramphenicol. We conclude that lambda-plasmid replication is under stringent control, probably as a result of the action of ppGpp, the indirect product of the relA gene, on RNA polymerase. The problem of stability of the lambda-O initiator protein is discussed.