DNA replication in Escherichia coli mutants that lack protein HU.

DNA replication in Escherichia coli cells lacking protein HU was studied. HU has been suggested to be involved in the initiation of replication from in vitro studies. The isolated HU mutants, however, are viable under normal growth conditions (M. Wada, Y. Kano, T. Ogawa, T. Okazaki, and F. Imamoto, J. Mol. Biol. 204:581-591, 1988). Chromosomal replication in the mutants appeared to be normal with respect to bidirectional replication from oriC and to its dependence on dnaA and some other dna gene products. No significant defect was observed in DNA synthesis in vitro with a crude enzyme fraction prepared from the mutant cells. These results, along with the earlier in vitro studies, suggest that other histonelike protein(s) may substitute for HU in the initiation of replication in the mutant cells. Minichromosomes were more unstable in the mutants. In the absence of either the mioC promoter, from which transcription enters oriC, or the DnaA box (DnaA protein-binding site) just upstream of the mioC promoter, the minichromosomes were especially unstable in the HU mutant and were integrated into the chromosomal oriC region under conditions selective for the plasmid-harboring cells.