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Abstract— Action spectra for lethality of both stationary and exponentially growing cells of recombinationless (recA) mutants of Salmonella typhimurium and Escherichia coli were obtained. Maximum sensitivity was observed at 260nm which corresponds to the maximum absorbance of DNA. However, a shoulder occurred in the 280–300 nm range that departed significantly from the absorption spectrum of DNA. At wavelengths longer than 320nm, the shapes of inactivation curves departed significantly from those at wavelengths shorter than 320nm and survival curves at wavelengths longer than 320nm had a large shoulder. A small peak or shoulder occurred in the 330–340nm region of the action spectra. The special sensitivity of recA mutants to broad spectrum near‐UV radiation may be due to synergistic effects of different wavelengths. Parallels between the inactivation of recA mutants and the induction of a photoproduct of l‐tryptophan toxic for recA mutants (now known to be H2O2) suggest that H2O2 photoproduct from endogenous tryptophan may be involved in the high sensitivity of these strains to broad spectrum near‐UV radiation.

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