Caffeine- and amino acid-effects upon try⁺ revertant yield in U.V.-irradiated hcr⁺ and hcr^- mutants of E. coli B/r

Summary

The influence of supplementation of the post-irradiation plating medium with caffeine and/or amino acids, upon both U.V.-induced killing and try⁺ reversion yield, has been studied in hcr⁺ and hcr^- derivatives of E. coli B/r tryptophan auxotroph WWP-2. All experiments have been carried out with stationary phase cells grown in aerated nutrient broth.

In the hcr^- strain, caffeine causes no enhancement of either killing or try⁺ revertant yield. There is mutational enhancement by supplementation with a low level of tryptophan, and an even greater effect when supplementation is with tryptophan plus an additional pool of other amino acids.

In the hcr⁺ strain, tryptophan and/or a pool of other amino acids cause an enhancement of try⁺ yield, as in the hcr^- strain. Caffeine causes lethal enhancement on several different media. There is an apparently straightforward dose enhancement by caffeine, of lethality and try⁺ revertant frequency, on media containing no, or only, tryptophan. On media containing, however, both a low level of tryptophan and an additional pool of other amino acids, caffeine causes a preferential enhancement of try⁺ revertant frequency over and above pure dose enhancement.

These results suggest that U.V. may cause two types of lesion. One type may lead only to mutation, and is repaired by both the caffeine-insensitive MFD, and the caffeine-sensitive hcr, dark repair systems. This first type of lesion is protected from repair by the presence of a pool of amino acids. The second type of lesion is hypothesised to be capable of causing either lethality or mutation, is repaired only by the hcr dark-repair system, and is not subject to protection by an amino acid pool.