Fox- and Fix- mutants of Anabaena 7120 defective in heterocyst development and nitrogen fixation

Abstract

Five transposon-generated mutants ofAnabaena 7120 have been isolated; they are defective either in heterocyst development or nitrogen fixation or in both. Mutant M 19 with Het - Fix phenotype incapable of heterocyst development and unable to fix dinitrogen even under anaerobic condition represents mutation of a regulatory gene controlling heterocyst differentiation and nitrogen fixation. Other mutants such as M9, M18, M20 and M21 with Het + Fox - genotype capable of nitrogen fixation only under anaerobic conditions suggest that their aerobic nitrogen fixation was affected indirectly by mutation to oxygen protective components of heterocyst. When mutant's filaments starved for fixed nitrogen, they fragmented and most of the filaments become unicellular/2-10 cells (except M21) with or without heterocyst. Mutant M9 is interesting because its fragmented filaments fail to differentiate new intercalary heterocysts upon transfer to the medium devoid of fixed nitrogen. This suggests for mutation to pattern forming gene whose product in vegetative cells is required for the development of intercalary heterocysts. Unlike wild type having a high rate of potosynthetic evolution in N 2 grown cultures, mutants defective in aerobic nitrogen fixation exhibited reduced level of O, evolution.