The activation of glutamine synthetase from the cyanobacterium Anabaena cylindrica by thioredoxin

Abstract The present communication defines the conditions under which thioredoxin activates glutamine synthetase from Anabaena cylindrica . Effects are obtained at pH values around neutrality, and the activation is affected by Mg²⁺ in the assays. The thioredoxin systems from A. cylindrica and spinach are functionally interchangeable in the activation of glutamine synthetase. The enzyme is efficiently activated by thioredoxin_m and also by thioredoxin_f, but at much higher concentrations. Thioredoxin_m has previously been shown to activate NADPH-dependent malate dehydrogenase and isocitrate dehydrogenase from cyanobacteria. It is speculated that thioredoxin_m plays a role in the differentiation of vegetative cells to heterocysts.