Thermosensitive mutants of Escherichia coli in which deoxyribonucleic acid replication is inhibited at high temperature have been assayed for their response to thymine deprivation at permissive and nonpermissive temperatures. The survival rate of mutants in which initiation of replication is defective at 42 C (dnaA and dnaC-D) is much higher at nonpermissive temperatures than at 32 C. By contrast, thymineless death is not hindered at 42 C in two elongation mutants (dnaE and dnaG). Bacterial strains belonging to the dnaB class exhibit both types of behavior. These results are in accordance with the model linking thymineless death to a disruption of an active replication fork. We discuss the possibility that the dnaB protein takes part in this process of cell decay.Thymine-requiring bacteria exhibit profound metabolic disorders when grown in the absence of this precursor. Although their ribonucleic acid and protein syntheses continue, the halt in deoxyribonucleic acid (DNA) replication leads to a rapid loss of their colony-forming ability (1). A similar behavior has been described for bacteria unable to synthesize another specific precursor of DNA, deoxyribose (19). Mutants of Escherichia coli in which DNA replication is thermosensitive and comes to an immediate halt after a shift to the restrictive temperature are also known to lose their ability to multiply after incubation at 42 C (3, 4, 6, 10). Moreover, polA-(2) and ligase-deficient mutants (18) are more sensitive to thymine deprivation than their wild-type counterparts. It seems, therefore, as if any interference with DNA elongation leads to bacterial death.In the present work we have examined whether the block in DNA replication observed at 42 C in DNA thermosensitive mutants of E. coli (25) alters the rate of the cell death induced by thymine deprivation. We can expect one of the following responses. (i) The rate of thymineless death is enhanced in these strains at 42 C. This would mean that temperature-induced death and thymineless death are independent phenomena with totally or partially additive effects. (ii) The kinetics of bacterial death at 32 C and 42 C are comparable. This would indicate that the processes leading to thymineless death and to the death of DNA thermosensitive mutants at 42 C share a common step. In both cases, we could conclude that the protein affected by the thermosensitive mutation is not directly involved in these processes. (iii) Bacterial death at 42 C is considerably lowered. This would mean either that denaturation of the thermosensitive gene product protects the cells against death, similar to chloramphenicol treatment (11), or that this protein is involved in the processes leading to death. MATERIALS AND METHODS Bacterial strains. The main features of the strains used are summarized in Table 1. When necessary, spontaneous Thymutants have been isolated by aminopterin selection (17, 23). As controls, we used both spontaneous revertants and thermoresistant P1 transductants. We used as reference strain a derivative of E. coli K-...

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The interference of dnaB mutations of Escherichia coli with thymineless death is described. All the isogenic Thy- dnaB mutants of E. coli we have tested show a remarkable immunity towards cell death induced by thymine deprivation at the nonpermissive temperature. We have also constructed and tested an isogenic double dnaB dnaG mutant. It loses its viability in the absence of thymine at both permissive and nonpermissive temperatures. The role of the dnaB gene product is discussed.