Rhodobacter capsulatus E1F1 grows phototrophically with nitrate as nitrogen source. Using primers designed for conserved motifs in bacterial assimilatory nitrate reductases, a 450-bp DNA was amplified by PCR and used for the screening of a genomic library. A cosmid carrying an insert with four SalI fragments of 2.8, 4.1, 4.5, and 5.8 kb was isolated, and DNA sequencing revealed that it contains a nitrate assimilation (nas) gene region, including the hcp gene coding for a hybrid cluster protein (HCP). Expression of hcp is probably regulated by a nitrite-sensitive repressor encoded by the adjacent nsrR gene. A His 6 -HCP was overproduced in Escherichia coli and purified. HCP contained about 6 iron and 4 labile sulfide atoms per molecule, in agreement with the presence of both [2Fe-2S] and [4Fe-2S-2O] clusters, and showed hydroxylamine reductase activity, forming ammonia in vitro with methyl viologen as reductant. The apparent K m values for NH 2 OH and methyl viologen were 1 mM and 7 M, respectively, at the pH and temperature optima (9.3 and 40°C). The activity was oxygen-sensitive and was inhibited by sulfide and iron reagents. R. capsulatus E1F1 grew phototrophically, but not heterotrophically, with 1 mM NH 2 OH as nitrogen source, and up to 10 mM NH 2 OH was taken up by anaerobic resting cells. Ammonium was transiently accumulated in the media, and its assimilation was prevented by L-methionine-D,L-sulfoximine, a glutamine synthetase inhibitor. In addition, hydroxylamine-or nitrite-grown cells showed the higher hydroxylamine reductase activities. However, R. capsulatus B10S, a strain lacking the whole hcp-nas region, did not grow with 1 mM NH 2 OH. Also, E. coli cells overproducing HCP tolerate hydroxylamine better during anaerobic growth. These results suggest that HCP is involved in assimilation of NH 2 OH, a toxic product that could be formed during nitrate assimilation, probably in the nitrite reduction step.Bacteria use nitrate as a nitrogen source for growth, as a terminal electron acceptor for anaerobic respiration, or as an electron sink for redox balancing. Three different types of bacterial nitrate-reducing systems have been described: the cytoplasmic assimilatory Nas, the membrane-bound respiratory Nar, and the periplasmic dissimilatory Nap (1, 2). Nitrate assimilation is a key process of the nitrogen cycle that has been an object of biochemical and genetic research in higher plants, fungi, algae, and cyanobacteria, although it has only been scarcely studied in other bacteria. It is firmly established that the assimilatory nitrate-reducing system consists of a nitrate transport system and two metalloenzymes, the assimilatory nitrate and nitrite reductases, which catalyze the stepwise reduction of nitrate to nitrite and ammonium (1-4). Nitrate assimilation is usually regulated by nitrate/nitrite induction and ammonium repression. Genes coding for the assimilatory nitrate-reducing systems are normally clustered and have been cloned in several bacteria. These gene clusters code for regulatory and struct...

The phototrophic bacterium Rhodobacter capsulatus E1F1 assimilates nitrate under anaerobic phototrophic growth conditions. A 17 kb DNA region encoding the nitrate assimilation (nas) system of this bacterium has been cloned and sequenced. This region includes the genes coding for a putative ABC (ATP-binding cassette)-type nitrate transporter (nasFED) and the structural genes for the enzymes nitrate reductase (nasA), nitrite reductase (nasB) and hydroxylamine reductase (hcp). Three genes code for putative regulatory proteins: a nitrite-sensitive repressor (nsrR), a transcription antiterminator (nasT) and a nitrate sensor (nasS). Other genes probably involved in nitrate assimilation are also present in this region. The sequence analysis of these genes and the biochemical properties of the purified nitrate, nitrite and hydroxylamine reductases are reviewed.

A nas gene region from Rhodobacter capsulatus E1F1 containing the putative nasB gene for nitrite reductase was previously cloned. The recombinant His(6)-NasB protein overproduced in E. coli showed nitrite reductase activity in vitro with both reduced methyl viologen and NADH as electron donors. The apparent K ( m ) values for nitrite and NADH were 0.5 mM and 20 microM, respectively, at the pH and temperature optima (pH 9 and 30 degrees C). The optical spectrum showed features that indicate the presence of FAD, iron-sulfur cluster and siroheme as prosthetic groups, and nitrite reductase activity was inhibited by sulfide and iron reagents. These results indicate that the phototrophic bacterium R. capsulatus E1F1 possesses an assimilatory NADH-nitrite reductase similar to that described in non-phototrophic organisms.

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