Abstract
ONE way of investigating the mechanism underlying a biological pattern is to try to alter that pattern with chemical agents. This approach has frequently yielded an embarras de richesses—many compounds having a diversity of actions with no obvious common factor. Examples are the animalising or vegetalising agents in sea urchin embryos (see ref. 1), and, more recently, compounds which alter the polar organisation or number of tentacles in Hydra2. We report an effect of a chemical on a pattern where the situation seems to be different. The tryptophan analogue L-7-azatryptophan (azatryptophan) causes a striking change in the heterocyst pattern in the blue-green alga Anabaena. It is the only compound of many we have tried which has this effect. Moreover, we have evidence against one interpretation of its action, that it produces defective proteins by substituting for tryptophan. This is encouraging because other possible mechanisms offer more hope of being experimentally accessible.
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