No abstract

SummaryEscherichia 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 a dimer or the ab 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 ab heterodimer or the b dimer.