Department of Biochemistry and Molecular Biology Harvard University, Cambridge, Massachusetts 02138.
In E. coli, replication initiates at a genetically unique origin, oriC. Rapidly growing cells contain multiple oriC copies. Initiation occurs synchronously, once and only once per cell cycle at all origins present. Secondary initiations are prevented by a sequestration process that acts uniquely on newly replicated origins, which are marked because they are hemimethylated at GATC sites. We report the identification of a gene required for sequestration and demonstrate that this gene, seqA, also serves as a negative modulator of the primary initiation process. All previously identified in vivo initiation factors play positive roles. Thus, precise control of replication initiation may involve a balance between positive and negative elements. We suggest that SeqA might be a cooperativity factor, acting to make the replication initiation process dependent upon cooperative interactions among components.