Insufficient levels of the nrdAB-encoded ribonucleotide reductase underlie the severe growth defect of the hda E-coli strain

The ATP-bound form of the Escherichia coli DnaA replication initiator protein remodels the chromosomal origin of replication, oriC, to load the replicative helicase. The primary mechanism for regulating the activity of DnaA involves the Hda and clamp proteins, which act together to dramatically stimulate the intrinsic DNA-dependent ATPase activity of DnaA via a process termed Regulatory Inactivation of DnaA. In addition to hyperinitiation, strains lacking hda function also exhibit cold sensitive growth at 30 degrees C. Strains impaired for the other regulators of initiation (i.e., seqA or datA) fail to exhibit cold sensitivity. The goal of this study was to gain insight into why loss of hda function impedes growth. We used a genetic approach to isolate 9 suppressors of hda cold sensitivity, and characterized the mechanistic basis by which these suppressors alleviated hda cold sensitivity. Taken together, our results provide strong support for the view that the fundamental defect associated with hda is diminished levels of DNA precursors, particularly dGTP and dATP. We discuss possible mechanisms by which the suppressors identified here may regulate dNTP pool size, as well as similarities in phenotypes between the hda strain and hda(+) strains exposed to the ribonucleotide reductase inhibitor hydroxyurea.