The genetic organization of the nirD
locus of Pseudomonas stutzeri ZoBell, necessary for a
catalytically
active cytochrome cd1 (EC 1.9.3.2), was
determined.
The locus comprises the unidirectionally transcribed open reading
frames
nirFDLGH, downstream of nirMC of the nir gene cluster,
and
immediately upstream of the
norCB operon encoding nitric oxide (NO)
reductase (EC 1.7.99.7). Notable sequence relatedness was found between
NirF and cytochrome cd1 (NirS), within
NirDLGH,
and between NirM and NirC, suggesting several gene duplication events
in
this region. The derived NirF protein (391 amino acids, Mr
43,137)
has 23.8% identity (51.1% overall similarity) with NirS, but lacks the
N-terminal heme-c-binding domain of NirS. Insertional mutagenesis of
the
five open reading frames resulted in the loss of respiratory nitrite
reductase
activity in vivo and in vitro. Mutant strains, when induced with
nitrate
for denitrification, synthesized a periplasmic cytochrome
cd1
lacking heme d1. The defect was caused
by the
inability of the cell to synthesize heme d1.
The nirD locus is proposed to encode a multimeric and
multifunctional
enzyme complex involved in the synthesis of heme d1.
Mutations in nirFDLGH lowered substantially the expression
level
of norCB. Nir- mutants, unable to generate NO in vivo, provide
indirect
evidence for an NO sensor and an inducer role of NO for its cognate
reductase.