Two genes, norB and norZ, encoding two independent @entity842 reductases have been identified in @entity18394 H16. norB and norZ predict polypeptides of 84.5 kDa with @entity535 sequence identity of 90%. While norB resides on the megaplasmid pHG1, the norZ gene is located on a chromosomal DNA fragment. @entity535 sequence analysis suggests that norB and norZ encode integral membrane proteins composed of 14 membrane-spanning helices. The region encompassing helices 3 to 14 shows similarity to the NorB subunit of common bacterial @entity842 reductases, including the positions of six strictly conserved @entity3793 residues. Unlike the Nor enzymes characterized so far from denitrifying bacteria, NorB and NorZ of A. eutrophus contain an @entity1472 -terminal extension which may form two additional helices connected by a hydrophilic loop of 203 @entity1522 . The presence of a NorB/NorZ-like protein was predicted from the genome sequence of the cyanobacterium @entity324 . While the common NorB of denitrifying bacteria is associated with a second cytochrome c subunit, encoded by the neighboring gene norC, the nor loci of A. eutrophus and @entity324 lack adjacent norC homologs. The physiological roles of norB and norZ in A. eutrophus were investigated with mutants disrupted in the two genes. Mutants bearing single-site deletions in norB or norZ were affected neither in aerobic nor in anaerobic growth with @entity1507 or @entity3542 as the terminal electron acceptor. Inactivation of both norB and norZ was lethal to the cells under anaerobic growth conditions. Anaerobic growth was restored in the double mutant by introducing either norB or norZ on a broad-host-range plasmid. These results show that the norB and norZ gene products are isofunctional and instrumental in denitrification.

Two isofunctional @entity842 reductases in XXXX H16.

@entity1522 :: ('CHEBI:33709', 'Chemical') :: ['amino acids'] @entity1472 :: ('CHEBI:46882', 'Chemical') :: ['amino'] @entity18394 :: ('106590', 'Species') :: ['Alcaligenes eutrophus'] @entity535 :: ('CHEBI:33704', 'Chemical') :: ['amino acid', 'Amino acid'] @entity3793 :: ('CHEBI:27570', 'Chemical') :: ['histidine'] @entity3542 :: ('CHEBI:16301', 'Chemical') :: ['nitrite'] @entity1507 :: ('CHEBI:48107', 'Chemical') :: ['nitrate'] @entity842 :: ('MESH:D009569', 'Chemical') :: ['nitric oxide'] @entity324 :: ('1148', 'Species') :: ['Synechocystis', 'Synechocystis sp. strain PCC6803']
@entity18394 :: (106590,Species) :: ['Alcaligenes eutrophus']
