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and PetH is a ferredoxin NADP รพ oxidoreductase, both integral components
of the nitrogen fixation pathway. The concomitant accumulation of these
proteins with GlbN further strengthens the involvement of GlbN in nitro-
gen metabolism ( Hill et al., 1996 ).
Analysis of the context of the glbN gene revealed that the sequence
located approximately 100 basepairs upstream of the start of the gene is sim-
ilar to the NtcA-binding domain found in glnA promoter of the fresh water
cyanobacterium Synechococcus elongatus ( Hill et al., 1996 ). The NtcA domain
acts as a global transcriptional activator for the regulation of the genes for
nitrogen metabolism in cyanobacteria ( Flores & Herrero, 1994 ). This
suggests that glbN is regulated by nitrogen metabolism in a manner similar
to nitrate reductase. Mobility shift analysis of the upstream sequence using
recombinant NtcA confirmed that this sequence binds the regulatory
protein ( Hill et al., 1996 ).
Given these results, the authors proposed a function for the globin of
N. commune . GlbN is predicted to serve as an oxygen scavenger, perhaps act-
ing as a delivery molecule for a terminal cytochrome oxidase complex, to aid
in ATP production during the anaerobic conditions that exist during nitro-
gen fixation ( Hill et al., 1996 ). Although this proposed function still requires
experimental validation, it does fit with the evidence thus far accumulated
for this protein.
Polyclonal antibodies raised against a recombinant form of GlbN were
used to stain ultrathin-sectioned cells of N. commune UTEX 584. With these
gold-labelled antibodies, the GlbN protein was localized to the peripheral
membrane of the cells via transmission electron microscopy ( Hill et al.,
1996 ). The presence of GlbN on the peripheral membrane is seen in both
heterocysts and vegetative cells. Subsequent fractionations of cellular com-
partments demonstrate that GlbN is a soluble protein with all cellular GlbN
found in aqueous fraction of the cell and no protein found within the
membrane fractions.
The polyclonal antibodies were also used to scan for cross-reactivity in a
series of cyanobacterial strains. No other species contained a cross-reacting
protein of equivalent molecular weight to the GlbN protein found in
N. commune UTEX 584; however, several species, including the diazotroph
nonheterocyst-forming filamentous Trichodesmium thiebautii , did show the
presence of a slightly larger (approximately 18 vs. 12 kDa) protein ( Hill
et al., 1996 ). This cross-reacting protein appears to be constitutively
expressed even under oxic conditions, whereas the GlbN of N. commune
UTEX 584 is only induced during anaerobic growth. Although it is possible
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