DNA-replication recovery inhibition and subsequent reinitiation in UV-radiation-damaged E. coli: a strategy for survival.
Using the incorporation of [14C]thymine to measure DNA accumulation, it was shown that exposure of the B/r strain of Escherichia coli to 10 J/m2 of ultraviolet radiation (UV) inhibits replication for about 20 min, but then resumption of replication occurs. Pulse-labelling with [3H]thymidine after exposure of the WT strain to this fluence confirmed the transient inhibition and recovery of DNA replication. After recovery, the rate of accumulation of DNA in the culture increases, to exceed that of the exponentially growing culture, so that eventually the amount of DNA almost equals that of the unirradiated culture. After a higher fluence (20 J/m2), an inhibition of replication recovery was revealed. This fluence delays the reinitiation of DNA accumulation in the culture, measured by [14C]thymine incorporation, for 25 min more, in addition to the 20-min recovery period. This finding was confirmed with pulse-labelling studies, which revealed that the higher exposure represses the rates of replication for 45 min before replication at the normal rate reinitiates in the culture. It was proposed that the inhibition of recovery revealed by these investigations is effected by the UV-induction of an active DNA-replication recovery-inhibition process. With the uvrA strain, rate studies revealed that 1.5 J/m2 of UV (a reduced fluence necessary because of the greater sensitivity of the strain) induces a transient inhibition of DNA replication, with considerable recovery following. Exposure to 3.0 J/m2 induces the transient inhibition of replication, followed by massive recovery inhibition after 20 min of incubation. With uvrA recA, both the lower and the higher fluence resulted in an immediate block of replication with no recovery, confirming the recA gene dependency of the recovery process. The decrease in rate of replication comparable to that seen in the uvrA strain after 20 min, and taken as evidence of the function of the recovery-inhibition process, was not seen. The evidence supports the concept that a process somehow triggered by higher UV fluences functions to repress replication temporarily, presumably allowing time for repair processes to take place before replication overruns closely linked pyrimidine dimers on opposite strands to create lethal lesions.
Wadsworth Center for Laboratories and Research, New York State Department of Health, Albany 12201-0509.
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Doudney
D004261:DNA Replication / Q000528:radiation effects*; D004269:DNA, Bacterial / Q000528:radiation effects*; D004307:Dose-Response Relationship, Radiation; D004926:Escherichia coli / Q000235:genetics / Q000528:radiation effects*; D005798:Genes, Bacterial; D009154:Mutation; D011938:Rec A Recombinases / Q000235:genetics; D013045:Species Specificity; D013936:Thymidine; D014466:Ultraviolet Rays
D004269:DNA, Bacterial; D011938:Rec A Recombinases; D013936:Thymidine