Loss of aerobic nitrogen fixation associated with kasugamycin resistance in the heterocystous nitrogen-fixing, streptomycin resistant blue-green alga Nostoc muscorum.

Heterocystous N2-fixing, Streptomycin resistant (Het (+)Nif(+)Str-R) Nostoc muscorum produces two groups of Kasugamycin resistant (Kas-R) phenotypes: one group showing normal heterocyst frequency and normal aerobic growth with N2 as a nitrogen source, the other group showing loss of growth with N2 as nitrogen source under aerobic growth conditions. Examination of strains showing Kas-R associated loss of aerobic N2 growth shows them to comprise three classes: Class A showing loss of both heterocyst and nitrogenase activity, class B showing formation of heterocyst but no nitrogenase activity and class C lacking heterocyst but containing microaerobic nitrogenase activity. The three classes of mutants revert to aerobic prototrophy with almost similar frequency of about 10(-6) suggesting them to have arisen as a result of single mutational events. Their reversion to prototrophy is simultaneously accompanied by the loss of Kas-R phenotype. All the phenotypic revertants in general appear more or less similar to the parental strain in respect to heterocyst formation, aerobic N2 growth and Kasugamycin sensitivity (Kas-S).