Characterization of the replication of Escherichia coli DNA in the absence of protein synthesis: Stable DNA replication

After inhibiting DNA synthesis in Escherichia coli, repeated cycles of chromosome replication can occur in the absence of protein synthesis. This “stable” replication requires the products of all of the known dna genes.Stable replication results from inhibiting DNA synthesis by treatment with naladixic acid, cytosine arabinoside or hydroxyurea; or by placing dnaB, dnaE or dnaG mutants at non-permissive temperatures. It also follows a “shift-up” into rich medium in which RNA and protein are synthesized more rapidly than DNA. Paradoxically, stable replication is induced also by treatment with concentrations of streptolydigin which do not inhibit DNA replication but temporarily and partially inhibit RNA and protein synthesis. During all of these treatments, some protein synthesis must occur.Stable replication is not immediately expressed after a short period of thymine starvation or streptolydigin treatment, but requires a subsequent period of protein synthesis. Once established, however, the stable replication state is permanent and will persist in the absence of protein synthesis or during normal growth.After stable replication has been determined by a period of DNA inhibition, it is possible to inactivate replication by heating dnaA, B, C, E and G temperature-sensitive mutants. However, resynthesis of these gene products in the presence of thymine and at the permissive temperature restores stable replication activity. Since restoration of activity can occur under normal growth conditions which do not induce stable replication, it was concluded that the dnaA, B, C, E and G gene products do not directly determine the stabilized character of the replication fork.A model is presented which attempts to explain the ability of different treatments to induce stable replication.