We examined effects on supercoiled DNA topology of DnaA protein, the initiator protein of chromosomal DNA replication in Escherichia coli. The activity was identified in an analysis of plasmid DNA incubated with DnaA protein and DNA topoisomerase I. In Superose 12 gel filtration chromatography, the activity coeluted with DnaA protein. Incubation of DnaA protein with DNA at temperatures over 24 °C was required for this activity, which was observed with either oriC plasmid or the replicative form I of φX174 with no DnaA box. As binding of ATP or ADP to DnaA protein prevented the activity of DnaA protein on DNA topology, binding of the adenine nucleotide may regulate the activity.
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