Control of heterocyst and nitrogenase synthesis in cyanobacteria

The development of the heterocyst by filamentous nitrogen-fixing cyanobacteria provides an attractive model system for studying cellular differentiation. Heterocyst synthesis is repressed by the presence of exogenous combined nitrogen. In this report, it is shown that the tryptophan analog, D,L-7-azatryptophan (Aza-T), is capable of relieving the repressive effect of exogenous NH4NO3 on heterocyst and nitrogenase synthesis. In nitrogen-fixing cultures, the presence of 20 micron Aza-T increases the heterocyst frequency twofold. The glutamate analog, L-methionine-D,L-sulfoximine (MSX), has also been shown to cause a derepression in the synthesis of heterocysts and nitrogenase. However, unlike MSX, Aza-T does not appear to exert its effects by inhibiting the activity of glutamine synthetase. Therefore, glutamine synthetase may not be the sole key to the derepression of heterocyst and nitrogenase development in the cyanobacteria. It is hoped that a study of Aza-T action may lead to the elucidation of a novel control mechanism.