RESISTANCE TO NITROPHENOLIC HERBICIDES AND METRONIDAZOLE IN THE CYANOBACTERIUM SYNECHOCYSTIS SP PCC6803 AS A RESULT OF THE INACTIVATION OF A NITROREDUCTASE-LIKE PROTEIN ENCODED BY DRGA GENE

Dinoseb is a herbicide known to inhibit photos!:stem II electron trans fer like DCMU, triazine and phenolic-type herbicides, The mutant Din7 of the cyanobacterium Synechocystis sp, PCC 6803, selected for resista nce to dinoseb, and the mutant Ins2, constructed by the insertion of t he kanamycin resistance cassette into the drg.4 gene, were cross-resis tant to other nitrophenolic herbicides (DNOC, 2,4-dinitrophenol) and t o the cell inhibitor metronidazole but not to the photosystem II inhib itors DCMU or ioxynil. The Din7 mutant had the same characteristics of photosystem II inhibition by dinoseb as the wild type. This result su ggested the existence of another site for dinoseb inhibition. The wild type cells modified dinoseb to a nontoxic product that gave an absorp tion spectrum similar to that of dithionite treated dinoseb containing reduced nitro groups. In contrast, the Din7 mutant did not modify din oseb, These phenomena were controlled by the drgA gene encoding a prot ein which showed similarity to several enzymes having nitroreductase a ctivity. The addition of superoxide dismutase to the medium relieved t he toxic effect of dinoseb in wild type cells but not in Din7. It is p roposed that in wild type cells of Synechocystis sp, PCC 6803 the DrgA protein is involved in detoxification of dinoseb via the reduction of the nitro group(s) and this process is accompanied by the formation o f toxic superoxide anions. Mutations blocking the activity of the DrgA protein lead to the development of resistance to nitrophenolic herbic ides and metronidazole. (C) 1998 Federation of European Biochemical So cieties.