A cyanobacterial mutant resistant against a bleaching herbicide

The nitrogen fixing cyanobacterium Nostoc muscorum has been found to be sensitive to the Herbicide SAN 6706 [4-chloro-5 (dimethylamino)-2-(a,a,a,-trifluoro-m-tolyl)-3-(2H) pyridazinon] at 30-45 microM within 15 min. The toxicity was more severe in combined nitrogen-free (Ncomb-free) medium than in a combined nitrogen medium; this enhancement was reversible by supplementation of the medium with 3 mM glucose or 5 microM ATP, serving as carbon and/or energy source in this organism. A mutant of this cyanobacterium resistant to 3 mM SAN 6706 has been isolated and characterized to perform nitrogenase activity in exogenous ATP supplemented Ncomb-free medium. However, it exhibited a moderate growth in combined nitrogen media in the absence of external ATP. The resistance factor is higher than 100. Simultaneously, this strain possesses a cross-resistance to methylamine, a well-known inhibitor of photophosphorylation, irrespective of the exogenous ATP supply. The behavior of the mutant suggests a defective phosphorylation in its photosynthetic system.