[Replication and expression of plasmid pBR 322 during discontinuous culture of a stringent and relaxed Escherichia coli strain]

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 micrograms/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.

[Synthesis of alkaline phosphatase in a stringent and a relaxed strain of Escherichia coli under amino acid and phosphate limitation]

Synthesis of alkaline phosphatase in Escherichia coli is derepressed under phosphate starvation in the stringent strain CP78 as well as in its relaxed counterpart CP79. During limitation of phosphate as well as of amino acids a decrease of enzyme activity is observed, especially in the relaxed strain, which can not produce ppGpp under this conditions. After phosphate limitation synthesis of ppGpp is not stimulated and the kinetics of RNA synthesis is similar in both strains. We suggest that ppGpp is not directly involved in the regulation of gene expression during phosphate starvation.

Replication of pBR322 DNA in stringent and relaxed strains of Escherichia coli

Synthesis of both chromosomal and plasmid (pBR322) DNA was measured in E. coli strains differing in their relA allele (relA+:CP78; relA:CP79). It was found that upon limitation of a required amino acid or after valine treatment to trigger a stringent response synthesis of pBR322 DNA was stimulated only in the relaxed strain and was inhibited in its stringent counterpart. The results suggest that replication of plasmid DNA is negatively controlled by the relA+ allele.