THYMINE‐DIMER AND ACTION‐SPECTRUM EVIDENCE FOR INDIRECT PHOTOREACTIVATION IN ESCHERICHIA COLI*

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Abstract
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.