Genetics of Nitrogen Fixation in the Blue-Green Alga, Nostoc Muscorum

Mutants of the heterocyst-forming, nitrogen-fixing (het⁺ nif⁺) blue-green alga, Nostoc muscorum, have been isolated which do not fix nitrogen or reduce acetylene. One such mutant with phenotype het⁻ nif⁻ was made resistant to streptomycin (1000 microgram/ml). The resulting strain (het⁻ nif⁻ St-R) was crossed with the heterocyst forming, nitrogen fixing, streptomycin sensitive [het⁺ nif⁺ St-s) parent under conditions which counterselected the latter. Recombinants were obtained with a frequency of up to 4.6 in 10⁵ colonies. The frequency of spontaneous mutations or revertants of each parent growing alone was 1 in 10⁷ or less (Stewart and Singh, 1975). The high yield of new genotype from the new culture is interpreted as evidence of nif gene transfer in this alga.

The isolated nif mutants fall into two classes, one with the phenotype het⁻ nif⁻, and the other with phenotype het⁺ nif⁺. Their behaviour under various growth conditions suggests: (i) active nitrogenase is not involved in regulation of heterocyst spacing pattern, and (ii) heterocyst and nitrogenase production is under positive control of a common regulatory gene in N. muscorum (Singh and Stewart, unpublished).