REGULATION BY AZIDE OF HETEROCYST AND NITROGENASE IN AZIDE-RESISTANT MUTANTS OF THE CYANOBACTERIUM, NOSTOC MUSCORUM

A novel class of azide-resistant mutants of N, muscorum is described in which azide caused inhibition of heterocyst differentiation and nitrogen fixation without causing inhibition of growth. The results indicate the utilization of azide, as a fixed nitrogen source, by the mutant strain. An increase in the ability to take up azide and in the phycocyanin/chlorophyll ratio following growth of the mutant in azide-containing medium are additional findings which support the conclusion that the mutant utilizes azide as a source of nitrogen. In the parental strain, Ca²⁺ -dependent and Mg²⁺ -dependent ATPases, and cellular nitrate reductase were inhibited by azide. The corresponding ATPases from the mutant strain were not inhibited by azide. There was evidence, in cell-free extracts, for an enzyme system which utilized azide as an electron acceptor and NADPH-ferredoxin as electron donor. The activity of this system was significantly higher (on a protein basis) in cells of the mutant grown on azide than in cells of either the parent or the mutant when grown on nitrate. It is suggested that the azide resistance of this class of mutant is due to a mutation which leads to azide resistant Ca²⁺ - and Mg²⁺ - ATPases. Such a mutation may allow an azide utilizing system, inherently present in both parent and mutant strains, to be expressed.