Differential regulation of nitrogenase activity by ionic and osmotic stresses and permeable sugars in the Cyanobacterium Anabaena sp. strain L-31

Abstract

Nitrogenase (acetylene reduction) activity in Anabaena sp. strain L-31 is significantly enhanced by the addition of sucrose, but is inhibited upon addition of sodium chloride. The positive effect of sucrose is not a general osmotic stress effect since non-permeable osmolytes (mannitol or polyethylene glycol (PEG)) do not influence nitrogenase activity. Unlike enteric bacteria, Anabaena cells take up and metabolise sucrose and incorporate products of its catabolism into proteins. Cultures inhibited in photosynthesis retain the ability to take up sucrose but do not show acetylene reduction activity, even when supplemented with sucrose. Addition of an inhibitor of transcription (rifampicin) or of a repressor of nitrogenase biosynthesis (ammonium chloride) abolishes the positive effect of sucrose on acetylene reduction activity. Cultures grown with permeable sugars (glucose, fructose and sucrose) show significantly higher levels of dinitrogenase reductase (Fe-protein of nitrogenase complex) while those grown with NaCl lack the protein. Fe-protein content is not affected by non-permeable solutes. Thus, exogenous sucrose elevates dinitrogenase reductase synthesis but does not appear to support the requirement of reductant for nitrogenase activity. The data substantiate our previous finding that the ionic and osmotic stresses differentially regulate cyanobacterial nitrogenase activity and explain the relatively superior osmotolerance of diazotrophic cyanobacterial strains, as compared with their sensitivity to salinity stress.