Nitrate reductase (NaR) of a strain of
Selenomonas ruminantium was purified, and the gene encoding NaR (
nar) was sequenced. The 6.4 kbp
nar gene consisted of
narG,
H,
J, and
I in this order. The deduced amino acid sequences of these subunits resembled those of membrane-bound nitrate reductase-A reported for
Escherichia coli. It was shown that
narG,
H,
J, and
I are transcribed as a single polycistronic message (
nar operon). The level of intracellular
nar-mRNA was higher when
S. ruminantium was grown with nitrate than when grown without nitrate, suggesting that
nar transcription is enhanced by nitrate. The level of
nar-mRNA, which was in parallel to the amount of NaR per cellular nitrogen, was suggested to be enhanced in response to the deficiency of energy and electron supply. Therefore, NaR synthesis in
S. ruminantium appeared to be regulated at the transcriptional level in response to the availability of energy and electrons.
S. ruminantium reduced nitrate and fumarate simultaneously with no significant effect of fumarate on
nar transcription. Addition of fumarate stimulated nitrate reduction, which was caused by increased cell growth because of increased acquirement of ATP via electron transport phosphorylation coupled with fumarate reduction.
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