In Escherichia coli, regulatory inactivation of the replication initiator DnaA occurs after initiation as a result of hydrolysis of bound ATP to ADP, but it has been unknown how DnaA is controlled to coordinate cell growth and chromosomal replication in Gram-positive bacteria such as Staphylococcus aureus. This study examined the roles of ATP binding and its hydrolysis in the regulation of the S. aureus DnaA activity. In vitro, S. aureus DnaA melted S. aureus oriC in the presence of ATP but not ADP by a mechanism independent of ATP hydrolysis. Unlike E. coli DnaA, binding of ADP to S. aureus DnaA was unstable. As a result, at physiological concentrations of ATP, ADP bound to S. aureus DnaA was rapidly exchanged for ATP, thereby regenerating the ability of DnaA to form the open complex in vitro. Therefore, we examined whether formation of ADP-DnaA participates in suppression of replication initiation in vivo. Induction of the R318H mutant of the AAA؉ sensor 2 protein, which has decreased intrinsic ATPase activity, caused over-initiation of chromosome replication in S. aureus, suggesting that formation of ADP-DnaA suppresses the initiation step in S. aureus. Together with the biochemical features of S. aureus DnaA, the weak ability to convert ATP-DnaA into ADP-DnaA and the instability of ADP-DnaA, these results suggest that there may be unidentified system(s) for reducing the cellular ratio of ATPDnaA to ADP-DnaA in S. aureus and thereby delaying the reinitiation of DNA replication.The initiation step of chromosome replication is an important set point for coordinating cell growth and chromosomal replication, and it is tightly regulated so that it takes place only once per cell cycle. DNA replication in bacteria starts when the cell mass reaches an appropriate size and at the appropriate time, which is controlled in part by the quantity and activity of initiator DnaA protein (1-3). In Escherichia coli, DnaA binds to 9-mer DnaA boxes in oriC, the origin of chromosome replication, and melts the duplex at AT-rich 13-mers adjacent to oriC. DnaA then directs loading of the DnaB replicative helicase, which depends on the DnaC helicase loader and priming by the DnaG primase. This is followed by the synthesis of DNA by the DNA polymerase III holoenzyme (4).DnaA is composed of four domains: N-terminal domains I and II are involved in DnaA oligomerization and DnaADnaB interaction, and they show a diversity of sequence among bacteria. Domain III contains AAAϩ motifs, and the C-terminal domain IV mediates DNA binding (3, 5). Biochemical and genetic studies in E. coli have revealed adenine nucleotide binding-mediated control of the DnaA activity (6 -11). Specifically, ATP-bound DnaA actively promotes replication initiation, whereas the ADP-bound form is inert (6, 7). ATP promotes the self-assembly of DnaA at replication origins via a conformational change to form a righthanded helical filament (8, 9). In E. coli, 70% to 80% of DnaA exists as the inactive ADP-bound form, and active ATPDnaA levels increase during the initiation ...