Freezing of Bacillus subtilis in liquid nitrogen results, upon thawing of the cells, in an enhanced deoxyribonucleoside triphosphate and reduced thymidine (Tdr) incorporation into cellular deoxyribonucleic acid (DNA). The DNA synthesized from thymidine triphosphate (TTP) was made by a "repair"-type system as determined by density transfer experiments. The mono-and diphosphate precursors were also incorporated by a "repair"-type synthesis. When Tdr was used as the radioactive precursor in the assay mixture, the product was only that expected from a semiconservative synthesis. Superlethal ultraviolet light exposure of the freeze-treated cells stimulated incorporation of phosphorylated precursors into DNA. Tdr uptake was greatly reduced by ultraviolet exposure, and only repair synthesis was observed. TTP and Tdr do not compete with one another in this system. The possibility that two DNA synthesizing systems exist in separate, nonmixing cellular compartments is considered.
Bacillus subtilis was exposed to ultraviolet light (UV) or X rays, and gene frequency analysis was used to study the location of initiation sites of postirradiation deoxyribonucleic acid (DNA) synthesis. It was found that DNA synthesis resumes primarily from the origin after UV exposure. With X irradiation, the origin is not selectively replicated. Elevated origin-to-terminal marker ratios observed after UV exposure of exponentially growing cells were interpreted as evidence for selective UV resistance of the replicative origin region of the bacterial chromosome. This laboratory has been engaged in studies aimed at defining the quantitative and regulatory aspects of chromosome replication after ultraviolet light (UV) or X-ray challenge to bacterial cells. By using a combination of density and radioisotopic labeling techniques, it was previously shown (4) that, when bulk deoxyribonucleic acid (DNA) synthesis re
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.