ACCELERATED PUBLICATION * To whom correspondence should be addressed. Some Properties of Dissimilatory Nitrate Reductases Lacking

AbstractNovel periplasmic and membrane-bound nitrate reductases lacking molybdenum and molybdenum cofactor were isolated from the vanadate-reducing bacterium Pseudomonas isachenkovii, and their properties were studied. Both enzymes have some unusual features, i.e., the individual subunits (130-kD subunit of the membrane-bound enzyme and monomeric 55-kD subunit of the periplasmic enzyme) possess their own nitrate reductase activity. In addition, both enzymes are highly thermostable, their temperature optimum being at 70-80°C, which is unexpectedly high for enzymes from mesophilic bacteria. Similarly to conventional molyb-denum-containing nitrate reductases, these isolated enzymes are very sensitive to low concentrations of cyanide and azide. During anaerobic cell growth on medium with nitrate and vanadate, nitrate consumption is followed by a period of vanadate dissimilation, and this period is associated with some structural reorganizations of the nitrate reductases. Key words: nitrate reductase, vanadium-containing enzymes, denitrification Nitrate reductase (NR) is a key enzyme for nitrate assimilation in eukaryotes [1] and for nitrate assimila-tion or dissimilation in prokaryotes [2, 3]. The molyb-denum dependence of NR was first mentioned in [4