An azide-resistant mutant of the blue-green algaNostoc muscorum producing heterocyst and nitrogenase in the presence of fixed inorganic nitrogen source

Abstract

The N₂, NO ⁻₃ , NO ⁻₂ , NH ⁺₄ and glutamine growing cultures of parentNostoc muscorum are found more or less equally sensitive to azide inhibition of growth. A mutant strain resistant to sodium azide was isolated from the parent strain in NO ⁻₃ medium and the two strains were compared with regard to their heterocyst formation and nitrogenase activity in NO ⁻₃ , NO ⁻₂ , NH ⁺₄ and glutamine media. While the parent strain stops production of both heterocyst and nitrogenase in all the fixed nitrogen media, the azide resistant strain forms both in the fixed inorganic nitrogen media but only heterocyst and no nitrogenase in the glutamine medium. Clearly a single genetic determinant of regulatory nature appears to mediate azide-resistance as well as relief of heterocyst and nitrogenase formation from inhibition by the fixed inorganic nitrogen source. The results of glutamine effect on the heterocyst and nitrogenase formation of the two strains indicate the operation of two levels of glutamine-sensitive regulation, one which operates through the common genetic determinant of heterocyst and nitrogenase regulation and the other exclusive to nitrogenase regulation. The in vivo functional nitrogenase does not appear to be the reason for azide-resistance and neither ammonia nor glutamine or its close metabolic product seems to function in the control of heterocyst spacing.