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For plasmids derived from bacteriophage λ, the initiation of bidirectional DNA replication from oriλ depends on the stimulation of transcription from the p R promoter by the host replication initiator protein DnaA. Certain Escherichia coli dnaA(ts) mutants cannot be transformed by wild-type λ plasmids even at the temperature permissive to cell growth. This plasmid–host incompatibility appeared to be due to inefficient stimulation of transcription from the p R promoter by the mutant DnaA protein. This paper shows that there is a second mechanism for the incompatibility between λ plasmids and dnaA(ts) hosts, exemplified in this study by the dnaA46 mutant. This is based on the competition between the λ P protein and the host DnaA and DnaC proteins for DnaB helicase. Both mechanisms must be operative for the incompatibility.
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