UNWINDING OF THE ESCHERICHIA-COLI ORIGIN OF REPLICATION (ORIC) CAN OCCUR IN THE ABSENCE OF INITIATION PROTEINS BUT IS STABILIZED BY DNAA AND HISTONE-LIKE PROTEINS IHF OR HU

The unwinding of the origin of replication (oriC) is a critical step i n initiation of DNA replication in Escherichia coli. Previous observat ions indicate that efficient unwinding of supercoiled plasmid template s containing oriC sequences requires the DnaA initiation protein and o ne or more accessory factors. The precise contribution of each protein to this process is unknown. Here, we demonstrate that unwinding can o ccur under physiological conditions at the same bases in oriC, in eith er the presence or the absence of initiation proteins, as detected by a single-strand specific nuclease, P1. This suggests that oriC unwindi ng is a spontaneous event determined solely by DNA sequence. DnaA and IHF, as part of a large nucleoprotein complex, may function to stabili ze the DNA strand opening prior to initiation of DNA replication. (C) 1998 Academic Press.