DnaA - Function

Function

Active DnaA is DnaA bound to ATP. Immediately after a cell has divided, the level of active DnaA is low. The formation of the oriC/DnaA complex and DNA unwinding requires ATP hydrolysis.

The oriC site in E. coli has three AT rich 13 base pair regions (DUEs) followed by four 9 bp regions. Around 10 dnaA molecules bind to the 9 bp regions, which wrap around the proteins causing the DNA at the AT-rich region to unwind. There are 8 dnaA binding sites within oriC, to which dnaA binds with differential affinity. When DNA replication is about to commence, dnaA occupies all of the high and low affinity binding sites. The denatured AT-rich region allows for the recruitment of DnaB (helicase), which complexes with DnaC (helicase loader). DnaC helps the helicase to bind to and to properly accommodate the ssDNA at the 13 bp region; this is accomplished by ATP hydrolysis, after which DnaC is released. Single-strand binding proteins (SSBs) stabilize the single DNA strands in order to maintain the replication bubble. DnaB is a 5'→3' helicase, so it travels on the lagging strand. It associates with DnaG (a primase) to form the only primer for the leading strand and to add RNA primers on the lagging strand. The interaction between DnaG and DnaB is necessary to control the longitude of Okazaki fragments on the lagging strand. DNA polymerase III is then able to start DNA replication.

DnaA contains two conserved regions: the first is located in the central part of the protein and corresponds to the ATP-binding domain, the second is located in the C-terminal half and could be involved in DNA-binding.