DNA-protein interaction at the origin of DNA replication of the plasmid pSC101

The initiation of DNA replication of the low copy number plasmid pSC101 is dependent on the dnaA initiator protein encoded by Escherichia coli. We have previously reported that the minimum essential replicon of the plasmid encodes a approximately 37.5 kd protein and that the protein is necessary along with host-encoded proteins for the replication of the plasmid chromosome. In this communication we show that the plasmid-encoded protein has sequence-specific DNA-binding activity. The protein binds cooperatively to the replication origin of pSC101. Using chemical and enzymatic probes we have determined the contact points of the protein with the DNA and the precise domain of the replication origin recognized by the 37.5 kd protein. The specificity of the DNA-protein interaction would suggest that the 37.5 kd protein may possibly function by guiding the replisome to the correct DNA sequence on the chromosome of pSC101.