A soluble enzyme system that specifically initiates λdy plasmid DNA replication at a bacteriophage λ replication origin [Wold et al. (1982) Proc. Natl. Acad. Sci. USA 79, 6176-6180] is also capable of replicating the single-stranded circular chromosomes of phages M13 and øX174 to a duplex form. This chain initiation on single-stranded templates is novel in that it is absolutely dependent on the λ O and P protein chromosomal initiators and on several Escherichia coli proteins that are known to function in the replication of the λ chromosome in vivo, including the host dnaB, dnaG (primase), dnaJ and dnaK replication proteins. Strand initiation occurs at multiple sites following an O and P protein-dependent pre-priming step in which the DNA is converted into an activated nucleoprotein complex containing the bacterial dnaB protein. We propose a scheme for the initiation of DNA synthesis on single-stranded templates in this enzyme system that may be relevant to strand initiation events that occur during replication of phage λ in vivo.
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