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The Counteraction by Cysteine of the Effects of X-Rays and of Tryptophan on the Action of Specific Suppressor Systems in Drosophila melanogaster^*

Henry L. Plaine

( Department of Biology, The Johns Hopkins University, Baltimore, Maryland)

1. L-Cysteine was fed to larvae carrying both a suppressor-erupt and a suppressor-tumor system, to test its effect in combination with x-ray treatments and with supplementary L-tryptophan in the diet. Cysteine itself had no effect on erupt eyes or on the incidence of melanotic tumors.

2. Fed before or after x-ray treatment, cysteine greatly reduced the radiation effect which blocks the action of the suppressor genes and thus engenders both tumors and erupt eyes. Cysteine was more effective in counteracting the erupt eye effect when fed before the x-ray treatment; but it appeared to be more effective against the tumorigenic effect if fed after the x-ray treatment.

3. The greatest protective effect was obtained when cysteine was fed both before and after the x-ray treatment, the incidence of tumors being reduced from 78 per cent to 13 per cent and that of erupt eyes from 90 per cent to 14 per cent when the larvae were x-rayed 24 hours after hatching.

4. Cysteine, to a considerable degree, counteracted the harmful effects of x-rays on viability, particularly those which occurred during the pupal period.

5. Cysteine likewise counteracted in all respects the effects of feeding supplementary tryptophan. The feeding of L-tryptophan (0.5 per cent) plus L-cysteine (0.1 per cent) led to a significant reduction in the incidence of tumors from 63 per cent to 24 per cent and in the incidence of erupt eyes from 52 per cent to 7 per cent. The toxicity of tryptophan, during the pupal stage, was greatly reduced when cysteine was added to the medium.

6. In all respects, the interaction of tryptophan with oxygen or cysteine is strikingly like that of x-rays with oxygen and cysteine. This might imply the operation of a similar mechanism in these cases.

^* This work was supported (in part) by a contract with the Atomic Energy Commission [AT (30-1)-1472].

Present address: Department of Zoology and Entomology, The Ohio State University, Columbus, Ohio.

Received 8/22/54.

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