Table 2 Conserved proteins encoded in representative nos gene clusters
Designation Present
in clade
(Predicted) cofactors
(Anticipated) properties and function
N 2 O reductase
NosZ
I, II
Binuclear Cu A and
tetranuclear Cu Z centre
Conventional N 2 O reductase
cNosZ
II
Binuclear Cu A and
tetranuclear Cu Z centre;
one haem c
Cytochrome c N 2 O reductase
Accessory proteins serving in respiratory electron transport and/or NosZ/cNosZ activity
maintenance (prevention of Cu Z * formation or Cu Z * rescue mechanism) and/or maturation of
the Nos system
NosD
I, II
None
Periplasmic interaction with the putative
ABC transporter NosFY
NosF
I, II
None
ATP-hydrolysing cytoplasmic component of
the putative ABC transporter NosDFY
NosY
I, II
None
Membrane-integral component of the
putative ABC transporter NosDFY
exhibiting six predicted membrane-spanning
segments
NosL
I, II
Cu
Copper assembly at active site of NosZ/
cNosZ
NosC
I, II
One haem c
Periplasmic redox partner of NosZ
containing a conserved haem c binding
motif (CXXCH)
NosR
I
Covalently bound FMN
and two [4Fe–4S]
clusters
a
Membrane-bound electron transport protein
NosX
I
FAD
Periplasmic FMN incorporation into NosR
NosB
II
None
Membrane-bound protein (four or six
transmembrane segments) of unknown
function, possibly serving as a scaffold to
organize other accessory Nos proteins in the
membrane
NosG
II
Four [4Fe–4S] clusters
Redox partner of NosH
NosH
II
Two [4Fe–4S] clusters
a
Membrane-bound protein; part of a
predicted NosGH complex
NosC1
II
One haem c
Periplasmic electron transport containing a
conserved haem c binding motif (CXXCH)
NosC2
II
One haem c
Periplasmic electron transport containing a
conserved haem c binding motif (CXXCH)
See Fig. 3 for illustrations
a The primary protein structure contains two additional and conserved cytoplasmic CX 3 CP motifs
that are potentially involved in binding a cofactor
Adapted from [19]
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