IHF and Fis as Cell Cycle Regulators: Activation of the Replication Origin and the Regulatory Cycle of the DnaA Initiator.

This review summarizes current knowledge about the mechanisms of timely binding and dissociation of two nucleoid proteins, IHF and Fis, which play fundamental roles in the initiation of chromosomal DNA replication in . Replication is initiated from a unique replication origin called and is tightly regulated so that it occurs only once per cell cycle. The timing of replication initiation at is rigidly controlled by the timely binding of the initiator protein DnaA and IHF to . The first part of this review presents up-to-date knowledge about the timely stabilization of -IHF binding at during replication initiation. Recent advances in our understanding of the genome-wide profile of cell cycle-coordinated IHF binding have revealed the -specific stabilization of IHF binding by ATP-DnaA oligomers at and by an initiation-specific IHF binding consensus sequence at . The second part of this review summarizes the mechanism of the timely regulation of DnaA activity via the chromosomal loci (DnaA-reactivating sequence 2) and . The timing of replication initiation at is controlled predominantly by the phosphorylated form of the adenosine nucleotide bound to DnaA, i.e., ATP-DnaA, but not ADP-ADP, is competent for initiation. Before initiation, increases the level of ATP-DnaA by stimulating the exchange of ADP for ATP on DnaA. This function is activated by the site-specific and timely binding of both IHF and Fis within . After initiation, another chromosomal locus, , which inactivates ATP-DnaA by stimulating ATP hydrolysis, is activated by the timely binding of IHF. A recent study has shown that ATP-DnaA oligomers formed at -Fis binding sites competitively dissociate Fis via negative feedback, whereas IHF regulation at and still remains to be investigated. This review summarizes the current knowledge about the specific role of IHF and Fis in the regulation of replication initiation and proposes a mechanism for the regulation of timely IHF binding and dissociation at and .