The covalent structure of the blue copper-containing nitrite reductase from Achromobacter xylosoxidans

The blue copper-containing nitrite reductase from Alcaligenes xylosoxidans: cloning of the nirA gene and characterization of the recombinant enzyme

The nirA gene encoding the blue dissimilatory nitrite reductase from Alcaligenes xylosoxidans has been cloned and sequenced. To our knowledge, this is the first report of the characterization of a gene encoding a blue copper-containing nitrite reductase. The deduced amino acid sequence exhibits a high degree of similarity to other copper-containing nitrite reductases from various bacterial sources. The full-length protein included a 24-amino-acid leader peptide. The nirA gene was overexpressed in Escherichia coli and was shown to be exported to the periplasm. Purification was achieved in a single step, and analysis of the recombinant Nir enzyme revealed that cleavage of the signal peptide occurred at a position identical to that for the native enzyme isolated from A. xylosoxidans. The recombinant Nir isolated directly was blue and trimeric and, on the basis of electron paramagnetic resonance spectroscopy and metal analysis, possessed only type 1 copper centers. This type 2-depleted enzyme preparation also had a low nitrite reductase enzyme activity. Incubation of the periplasmic fraction with copper sulfate prior to purification resulted in the isolation of an enzyme with a full complement of type 1 and type 2 copper centers and a high specific activity. The kinetic properties of the recombinant enzyme were indistinguishable from those of the native nitrite reductase isolated from A. xylosoxidans. This rapid isolation procedure will greatly facilitate genetic and biochemical characterization of both wild-type and mutant derivatives of this protein.

Cloning, sequencing, and transcriptional studies of the gene encoding copper-containing nitrite reductase from Alcaligenes xylosoxidans NCIMB 11015

Gene encoding of the blue copper-containing nitrite reductase (nir) from Alcaligenes xylosoxidans NCIMB 11015 has been cloned and characterized. The nir is translated into a polypeptide of 360 amino acid residues as a precursor, and the N-terminal 24 residues are subsequently removed upon transport into the periplasm as a mature protein. A specific transcription product of nir was detected only in the presence of nitrate. The aeration level of the culture medium did not show a significant effect on the transcriptional level. A varsigma54 binding sequence is identified upstream of the transcriptional initiation at 53 to 26 nucleotides. A putative fnr box has also been identified in the sequence of the upstream region. The mature polypeptide showed 70% sequence identity with those of the Achromobacter cycloclastes enzyme. The transcriptional start point has been determined at 92 nucleotides upstream of the initiation codon and is preceded by the binding sites for varsigma54 and the fnr box. These results suggest that gene expression depends on the presence of nitrate and is stimulated under an anaerobic environment.