Summary: A pleiotropic dinitrogen and nitrate assimilation mutant was obtained by mutagenesis of sp. strain ATCC 29133 with -methyl--nitro--nitrosoguanidine followed by penicillin counterselection in the presence of NO and N. Mutant strain UCD 223 was capable of reducing acetylene in the free-living growth state only under anaerobic conditions, or under atmospheric conditions when in symbiotic association with Heterocysts of strain UCD 223 were noticeably lacking the cohesive outer polysaccharide layer of wild-type heterocysts. Oxygen microelectrode profiles of symbiotic tissue revealed an anaerobic environment in the symbiotic cavities containing The acetylene-reducing activities of strain UCD 223, and of its spontaneously-arising Fix revertant strain UCD 236, were not repressed by the presence of 10 m-NO when in the free-living growth state, in contrast to wild-type ATCC 29133. However, in activities of acetylene reduction by symbiotically associated ATCC 29133 and strains UCD 223 and UCD 236 were repressed by the presence of 10 m-NO . It appears that the symbiotic cavities of can physiologically replace the function of the heterocyst outer wall and that the repression of nitrogenase activity in symbiotic in response to the presence of NO , and probably NH , is mediated by


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