The synthesis of a functional nitrous oxide reductase requires an
assembly apparatus for the insertion of the prosthetic copper. Part of
the system is encoded by maturation genes located in Pseudomonas
stutzeri immediately downstream of the structural gene for the
enzyme. We have studied the transcriptional organization and regulation
of this region and found a nosDFYLtatE operon structure. In
addition to a putative ABC transporter, consisting of NosD, NosF, and
NosY, the operon encodes a Cu chaperone, NosL, and a component of the
Tat translocon, TatE. The nosD operon was activated in response
to anaerobiosis and nitrate denitrification. The membrane-bound
regulator NosR was required for operon expression; in addition, DnrD, a
regulator of the Crp-Fnr family, enhanced expression under anaerobic
conditions. This establishes a likely signal transduction sequence of
NO -> DnrD ->
nosR/NosR-> nosD operon.
DnrD-dependent
expression was also observed for the nnrS operon (located
immediately downstream of the nosD operon), which encodes a
putative heme-Cu protein (NnrS) and a member of the short-chain
dehydrogenase family (ORF247). The NosF protein, encoded within the nosD
operon, exhibits sequence similarity to ABC-type ATPases. It was fused
to the Escherichia coli maltose-binding protein and
overexpressed
in soluble form. The fusion protein was purified and shown to have
ATPase activity. NosF is the first maturation factor for which a
catalytic function has been demonstrated in vitro.