Twelve amino acid analogs and related compounds were screened for their ability to induce heterocysts in ammonia-repressed, undifferentiated filaments of Anabaena variabilis. As has been previously described, 1-methionine-dl-sulfoximine induces both heterocysts and nitrogenase. In contrast, dl-7-azatryptophan and fl-2-thienyl-dl-alanine were found to induce heterocysts but not nitrogenase activity (measured as acetylene reduction) even under microaerobic conditions. When the initial ammonium

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... centration was reduced, dl-7-azatryptophantreated cultures sequentially produced heterocysts and then nitrogenase activity, but nitrogenase was detected only when a parallel culture without analog also became capable of acetylene reduction. Neither of the two latter analogs affected y-glutamyl transferase activity in crude extracts. All three analogs significantly reduced the mean interheterocyst distance in nitrogen-fixing cultures.