Figure 5.

Degradation or processing of the nascent DNA at blocked replication forks following UV irradiation in various mutants. (A) A 10 s pulse of [³H]thymidine is added to [¹⁴C]thymine-prelabeled cells immediately before the cells are filtered and irradiated with 25 J/m² in nonlabeled medium. To assay for degradation, the fraction of radioactivity remaining in the DNA, as measured by TCA precipitation, is plotted against time. (B) The loss (or degradation) of ¹⁴C genomic DNA (open symbols) can be compared with the loss of the ³H DNA synthesized at the growing fork just before irradiation (closed symbols). In wild-type cells, no DNA degradation is detected in the absence of UV irradiation. Following UV irradiation, limited degradation at the growing fork is detected at times before the recovery of replication. (C) In recA mutants, both the nascent DNA and the entire genomic DNA is rapidly degraded. In recF mutants, approximately half of the pulse-labeled nascent DNA is degraded. In uvrA mutants, the nascent DNA degradation is similar in extent to that in wild-type cells. In recJ or recQ mutants, no detectable degradation of nascent DNA occurs. In recBC mutants, the nascent DNA degradation is similar in extent to that in wild-type cells. [Adapted with permission from ref. 45 (Copyright 1999, Am. Soc. Microbiol.) and from Mol. Gen. Genet., “RecQ and RecJ process blocked replication forks prior to the resumption of replication in UV-irradiated Escherichia coli,” J. Courcelle & P. C. Hanawalt, 262, pp. 543–551, figures 2 and 3 (Copyright 1999, Springer-Verlag; ref. 53).]