Roles of Escherichia coli histone-like protein HU in DNA replication: HU-beta suppresses the thermosensitivity of dnaA46ts.

The HU protein is a small, basic, heat-stable DNA-binding protein that is well-conserved in prokaryotes and is associated with the bacterial nucleoid. In enterobacteria, including Escherichia coli, HU is a heterotypic dimer, HUalphabeta, composed of two closely related sub-units encoded by the hupA and hupB genes, respectively. HU was shown to participate in vitro in the initiation of DNA replication as an accessory factor to assist the action of DnaA protein in the unwinding of oriC DNA. To further elucidate the role of HU in the regulation of the DNA replication initiation process, we tested the synchrony phenotype in the absence of either one or both HU sub-units. The hupAB mutant exhibits an asynchronous initiation, the hupA mutant shows a similar reduced synchrony, whereas the hupB mutant shows a normal phenotype. Using a thermosensitive dnaA46 strain (dnaA46ts), an initiation mutant, we reveal a special role of HUbeta. The presence of a plasmid overproducing HUbeta in a dnaA46ts lacking HU (hupAB background) compensates for the thermosensitivity of this initiation mutant. Moreover, the overproduction of HUbeta confers to dnaA46ts a pattern of asynchrony similar to that of a dnaAcos, the intragenic suppressor of dnaA46ts. We show that the relative ratio of HUalpha versus HUbeta is greatly perturbed in dnaA46ts which accumulates little, if any, HUbeta. Therefore, the suppression of thermosensitivity in dnaA46hupAB by HUbeta may be caused by an unexpected absence of HUbeta in the dnaA46ts mutant. Visibly the HU composition is sensitive to the different states of DnaA, and may play a role during the regulation of the initiation process of the DNA replication by affecting subsequent events along the cell cycle.