Escherichia coli DnaA interacts with HU in initiation at the E-coli replication origin

Escherichia coli HU protein is a dimer encoded by two closely related genes whose expression is growth phase-dependent. As a major component of the bacterial nucleoid, HU binds to DNA non-specifically, but acts at the chromosomal origin (oriC) during initiation by stimulating strand opening in vitro. We show that the a dimer of HU is more active than other forms of HU in initiation of an oriC-containing plasmid because it more effectively promotes strand opening of oriC. Other results demonstrate that HU stabilizes the DnaA oligomer bound to oriC, and that the a subunit of HU interacts with the N-terminal region of DnaA. These observations support a model whereby DnaA interacts with the alpha dimer or the alpha beta heterodimer, depending on their cellular abundance, to recruit the respective form of HU to oriC. The greater activity of the a dimer of HU at oriC may stimulate initiation during early log phase compared with the lesser activity of the alpha beta heterodimer or the beta dimer.