Abstract— Cells of Escherichia coli B and B phr‐ were labeled with tritiated thymidine, exposed to inactivating ultraviolet radiation at 254 nm, given photoreactivation (PR) treatment immediately thereafter, and then immediately hydrolyzed and assayed for thymine‐containing dimers. It was found that (1) PR treatment of strain B phr‐ does not split thymine dimers and (2) the amount of splitting of thymine dimers in strain B at 334 nm is only 45 per cent of the amount of splitting observed at 405 nm for the same amount of biological PR. These findings show that all of the PR in E. coli B phr‐, and part of the PR at 334 nm in E. coli B, is indirect (does not use PR enzyme) and is not due to thymine‐dimer splitting. Action spectra for PR in Escherichia' coli strains Bs‐1 and B/r were obtained. At wavelengths below 366 nm (the indirect PR region), PR is relatively much more efficient in strain B/r in logarithmic phase than in strain B/r in stationary phase or in strain Bs‐l. This is consistent with the expectation that indirect PR would not be exhibited by strains, such as Bs‐1 that lack dark‐repair ability, or by certain strains in the stationary phase, such as B/r, which, upon plating, have a ‘built‐in’ growth delay that can permit optimal dark repair, but indirect PR would be exhibited by intermediate cases, such as in strain B (which is less resistant to u.v. than strain B/r) or in strain B/r in logarithmic phase. These findings support the hypothesis that indirect PR results from enhancement of dark repair.
An action spectrum was obtained for photoreactivation of killing (PR) of Streptomyces griseus conidia. This spectrum shows a major peak around 436 nm, originally observed by A. Kelner, and a secondary peak at 3 I3 nm not previously reported. The rate of PR shows a strong dependence upon temperature and dose rate of the PR light at 436 nm, but this decreases to only a slight dependence upon these parameters at 3 I3 nm. These findings suggested that PR at 436 nm in this organism is of the usual photoenzymatic type, but that PR at 313 nm might be of a different kind.A mutant (PHR-I) of S . griseus was found that shows only a narrow range of PR (roughly 3 10-400 nm) with a single peak at 3 I3 nm. The PR efficiency was lower than for wild type and the PR sector not greater than one-half that of wild type. This PR shows no temperature dependence. Essentially similar behavior was observed with wild-type Streptomyces coelicolor. These findings show that at least some of the PR at 3 I3 nm is a separable phenomenon. It is therefore unlikely to involve a mechanism identical to that at 436 nm.The nature of PR at 3 I3 nm in Streptomyces is not known. If it is enzymatic, it is remarkable in having little or no dependence upon temperature and dose rate. Absence of photoprotection and liquid-holding recovery indicate that it is not indirect PR. Some of it (that part exhibited by S. griseus PHR-I and S. coelicolor) might result from a direct photochemical action on DNA. I N T R O D U C T I O NPH~TOREACTIVATION (PR) was discovered in Streptomyces griseus by Kelner [ 11, who observed that the PR rate showed a strong dependence upon temperature during PR and that dose-rate-saturation effects were observed (failure of Bunsen-Roscoe reciprocity of time and dose rate). Early workers quickly concluded that PR was mediated primarily by an enzyme that acted upon DNA. PR enzyme activity has now been found in extracts from several sources, and the enzyme from baker's yeast has been considerably purified [2]. In this paper we shall call this enzymatic photoreactivation 'Type I PR'. In Escherichia coli, Type 1 PR (believed to be the only type in E. coli B,-,[3]) is characterized by efficient PR in the range 320-440 nm, with a peak activity around 385 nm (see Fig. 8). The same is true for the yeast PR enzyme. In Neurosporu crussu, however, the range of efficient PR is slightly broader, and shows a peak around 400 nm (Fig. 8).In 1965, Jagger and StafFord[4] presented evidence for a second kind of photoreactivation in E. coli B. We shall call this 'Type I I PR'. It is characterized by a much narrower wavelength range (3 10-380 nm) with a peak of activity around 340 nm. That Type I I PR differs from Type I PR is demonstrated by (1) a complete lack of dose-rate dependence and only slight temperature dependence[4], (2) its occurrence [4] in a strain lacking PR-enzyme activity in cell extracts [5,6], and (3) the fact that this kind of PR
Abstract— From the rates of cyclobutyl dipyrimidine (Pyr < > Pyr) formation and the ratio of inactivation of transforming or phage DNA caused by direct (254 nm) or sensitized (1.0 M acetone, 313 nm; 0.02 M acetophenone, 334 nm) irradiation, we conclude that Thy < > Pyr and Cyt < > Pyr are equally lethal, and that they are repaired with equal efficiency by the host cell. Not all the damage formed by photosensitized irradlation can be photoenzymatically repaired, especially when acetone is the sensitizer. We found no compelling evidence for photosensitized interstrand cross‐links or sensitizer‐DNA addition products for the fluence range used in these studies (< 106 Jm‐2); moreover, strand breakage can account for only a part of the non‐photorepairable damage. We suggest that a fraction of the damage may be due to Pyr < > Pyr isomers other than the cis, syn type usually formed in native DNA by far‐UV light.
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