The regulatory element NosR was
identified
within the nos region of the denitrification gene cluster of Pseudomonas
stutzeri ZoBell (ATCC 14405) and characterized. It is essential for
expression of the N2O reductase encoded by nosZ
immediately downstream of nosR. The nosR region was
initially
identified by Tn5 mutagenesis (W. G. Zumft, K. Döhler, and
H. Körner, J. Bacteriol. 163:918-924, 1985). It consists of a
single
open reading frame of 2,172 nucleotides and has the coding capacity for
an 81.9-kDa protein. The codon usage for
nosR, with its high G+C
content of 62.4 mol% and a preference for G or C at the third position,
is characteristic for a Pseudomonas gene. Hydropathy analysis
classified
NosR as an integral membrane protein with at least seven
membrane-spanning
segments. No similarity to known bacterial regulator proteins was found
in a data bank search. However, the C terminus of NosR shows sequence
similarity
to the cysteine clusters of several 2[4Fe-4S] bacterial ferredoxins. A
monocistronic mRNA for nosZ which allowed us to monitor NosR
function
was identified. Complementation of Nos- mutant MK418 (nosR::Tn5)
with the nosR gene supplied in trans restored nosZ
transcription and expression of a catalytically active N2O
reductase. In addition to evidence of the requirement for NosR,
indirect
evidence for involvement of the transcriptional regulator Fnr is
presented.