By transforming N2O to N2,
the multicopper enzyme nitrous oxide reductase provides a periplasmic
electron
sink for a respiratory chain that is part of denitrification. The
signal
sequence of the enzyme carries the heptameric twin-arginine consensus
motif
characteristic of the Tat pathway. We have identified tat genes of
Pseudomonas
stutzeri and functionally analyzed the unlinked tatC and tatE loci. A
tatC
mutant retained N2O reductase in the cytoplasm
in
the unprocessed form and lacking the metal cofactors. This is contrary
to viewing the Tat system as specific only for fully assembled
proteins.
A C618V exchange in the electron transfer center CuA
rendered the enzyme largely incompetent for transport. The location of
the mutation in the C-terminal domain of N2O
reductase
implies that the Tat system acts on a completely synthesized protein
and
is sensitive to a late structural variation in folding. By generating a
tatE mutant and a reductase-overproducing strain, we show a function
for
TatE in N2O reductase translocation. Further, we
have
found that the Tat and Sec pathways have to cooperate to produce a
functional
nitrite reductase system. The cytochrome cd1 nitrite reductase was
found
in the periplasm of the tatC mutant, suggesting export by the Sec
pathway;
however, the enzyme lacked the heme D1 macrocycle.
The NirD protein as part of a complex required for heme D1
synthesis or processing carries a putative Tat signal peptide. Since NO
reduction was also inhibited in the tatC mutant, the Tat protein
translocation
system is necessary in multiple ways for establishing anaerobic nitrite
denitrification.