Heterocyst differentiation and tryptophan metabolism in the cyanobacterium Anabaena sp. CA

Anabaena sp. CA does not synthesize heterocysts or express nitrogenase activity when grown with nitrate as the nitrogen source. Heterocysts and nitrogenase are induced in such cultures by various tryptophan analogs. The effect does not require inhibition of de novo protein synthesis in the culture. It is restricted to tryptophan analogs only, and, more specifically, to those which can be incorporated into proteins. dl-7-Azatryptophan was effective at triggering both heterocysts and nitrogenase when incubated in the culture for only 1–2 h, even though 6–7 h was required for heterocysts to fully mature and nitrogenase activity to be expressed. Chloramphenicol completely negated this effect, supporting the idea that the analogs are either incorporated into protein themselves or trigger the synthesis of proteins which initiate complete development of mature heterocysts. Using toluene-permeabilized cells, we have shown that anthranilate synthetase, the first key enzyme in tryptophan biosynthesis, has glutamine-dependent activity. This activity can be effectively feedback inhibited by the various tryptophan analogs at concentrations which are also effective in triggering heterocyst differentiation. These data provide firm evidence for a link between tryptophan biosynthesis, nitrogenase synthesis, heterocyst differentiation, and primary ammonia assimilation.