Distribution of thioredoxins in heterocysts and vegetative cells of cyanobacteria

Abstract

Thioredoxin m from the heterocystous cyanobacterium Anabaena variabilis was purified to homogeneity. The protein activated NADP +-dependent malate dehydrogenase from spinach but hardly at all from Anabaena variabilis. Using polyclonal antibodies against thioredoxin m, Western blot analysis showed that the protein occurs in both heterocysts and vegetative cells of Anabaena variabilis Anabaena 7119 and Anabaena 7120, but not in Anabaena cylindrica. Two precipitation bands in the Western blots were observed with extracts from the unicellular Anacystis nidulans, a weak band with extracts from Escherichia coli, and none in the case of Cyanophora paradoxa. The occurrence of thioredoxin m in heterocysts and vegetative cells was shown by (a) assaying different concentrations of extracts from both cell types in the thioredoxin-dependent activation of NADP +-dependent malate dehydrogenase from spinach; (b) determining the thioredoxin m content by the enzyme linked immunosorbent assay (ELISA) technique; (c) immuno-electron microscopy. It has been hypothesized that glucose-6-P-dehydrogenase and the hexose monophosphate shunt (as the main pathway for the generation of reluctant in heterocysts) are active during light-dependent nitrogen fixation due to the absence (or reduced level) of thioredoxin. Such an idea is not supported at all by the data of this investigation. The present findings are faced with the problem how the inactivation of glucose-6-P-dehydrogenase by thioredoxin is prevented in heterocysts. This investigation confirms that thioredoxin f-like proteins with unusually high molecular weights occur in extracts from cyanobacteria. These proteins only activate fructose-1,6-bisphosphatase from spinach but not the homologous enzyme or any other protein tested from cyanobacteria. Thus the functions of these proteins remain unknown.