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 a cid-starved relA mutants (relaxed response). This replication is perpe tuated by the replication complex containing the lambda O protein (whi ch is protected from proteases by other elements of the complex) and i nherited by one of two daughter copies after a replication round, Sinc e a fraction of stable lambda O protein was observed in relA(-) and re lA(+) strains, and negative regulation by the lambda Cro repressor doe s not seem to be important in the stringent or relaxed response of lam bda plasmid replication to amino acid starvation, the inhibition of la mbda, plasmid replication in amino acid-starved wild type strains was investigated, lambda plasmids were unable to replicate in amino acid-s tarved relA(-) bacteria treated with rifampicin, Moreover, transcripti on 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 cell s containing increased levels of ppGpp, However, it was little or tota lly not affected by the relaxed response, The replacement of pR with p lac (which is known to be uninhibited by ppGpp) in a lambda. plasmid r esulted in its DNA replication during relaxed and stringent responses as well as during overproduction of ppGpp in unstarved bacteria, We co nclude that ppGpp-mediated inhibition of transcriptional activation of ori lambda is responsible for inhibition of lambda plasmid DNA replic ation in amino acid-starved wild type strains, A model of stringent co ntrol of lambda plasmid DNA replication is proposed and an analogy wit h eukaryotic DNA replication suggested.