THE MECHANISM OF THE STRINGENT CONTROL OF LAMBDA-PLASMID DNA-REPLICATION

lambda plasmid DNA replication is inhibited in amino acid-starved wild type Escherichia coli strains (stringent response) but not in amino acid-starved relA mutants (relaxed response). This replication is perpetuated by the replication complex containing the lambda O protein (which is protected from proteases by other elements of the complex) and inherited by one of two daughter copies after a replication round, Since a fraction of stable lambda O protein was observed in relA(-) and relA(+) strains, and negative regulation by the lambda Cro repressor does not seem to be important in the stringent or relaxed response of lambda plasmid replication to amino acid starvation, the inhibition of lambda, plasmid replication in amino acid-starved wild type strains was investigated, lambda plasmids were unable to replicate in amino acid-starved relA(-) bacteria treated with rifampicin, Moreover, transcription from PR, which produces mRNA for replication protein synthesis and serves as transcriptional activation of ori lambda, was significantly decreased during the stringent response as well as in non-starved cells containing increased levels of ppGpp, However, it was little or totally not affected by the relaxed response, The replacement of pR with plac (which is known to be uninhibited by ppGpp) in a lambda. plasmid resulted in its DNA replication during relaxed and stringent responses as well as during overproduction of ppGpp in unstarved bacteria, We conclude that ppGpp-mediated inhibition of transcriptional activation of ori lambda is responsible for inhibition of lambda plasmid DNA replication in amino acid-starved wild type strains, A model of stringent control of lambda plasmid DNA replication is proposed and an analogy with eukaryotic DNA replication suggested.