A cyanobacterial mutant resistant against a bleaching herbicide

The nitrogen fixing cyanobacterium Nostoc muscorum has been found to be sen sitive to the herbicide SAN 6706 [4-chloro-5 (dimethylamino)-2-(a,a,a,-trif luoro-m-tolyl)-3-(2H) pyridazinon] at 30-45 mu M within 15 min. The toxicit y was more severe in combined nitrogen-free (N-comb-free) medium than in a combined nitrogen medium; this enhancement was reversible by supplementatio n of the medium with 3 mM glucose or 5 mu M ATP, serving as carbon and/or e nergy source in this organism. A mutant of this cyanobacterium resistant to 3 mM SAN 6706 has been isolated and characterized to perform nitrogenase a ctivity in exogenous ATP supplemented N-comb-free medium. However, it exhib ited a moderate growth in combined nitrogen media in the absence of externa l ATP. The resistance factor is higher than 100. Simultaneously, this strai n possesses a cross-resistance to methylamine, a well-known inhibitor of ph otophosphorylation, irrespective of the exogenous ATP supply. The behavior of the mutant suggests a defective phosphorylation in its photosynthetic sy stem.