Ribonucleoside diphosphate reductase is a component of the replication hyperstructure in Escherichia coli

Although the nrdA101 allele codes for a ribonucleoside diphosphate (rNDP) reductase that is essentially destroyed in less than 2 min at 42°C, and chemical inhibition of the enzyme by hydroxyurea stops DNA synthesis at once, we found that incubation at 42°C of an Escherichia coli strain containing this allele allows DNA replication for about 40 min. This suggests that mutant rNDP reductase is protected from thermal inactivation by some hyperstructure. If, to‐gether with the temperature upshift, RNA or protein synthesis is inhibited, the thermostability time of the mutant rNDP reductase becomes at least as long as the replication time and residual DNA synthesis becomes a run‐out replication producing fully replicated chromosomes. This suggests that cessation of replication in the nrdA101 mutant strain is not the result of inactivation of its gene product but of the activity of a protein reflecting the presence of a partially altered enzyme. The absence of Tus protein, which specifically stops the replication complex by inhibiting replicative helicase activity, allows forks to replicate for a longer time at the restrictive temperature in the nrdA101 mutant strain. We therefore propose that rNDP reductase is a component of the replication complex, and that this association with other proteins protects the protein coded by allele nrdA101 from thermal inactivation.