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Applied and Environmental Microbiology, December 2003, p. 7009-7018, Vol. 69, No. 12
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.12.7009-7018.2003
Copyright © 2003, American
Society for
Microbiology. All Rights Reserved.
Nitrate/Nitrite Assimilation System of the Marine Picoplanktonic Cyanobacterium Synechococcus sp. Strain WH 8103: Effect of Nitrogen Source and Availability on Gene Expression
Clare Bird and Michael Wyman*
School
of Biological and Environmental Sciences, University of Stirling,
Stirling FK9 4LA, United Kingdom
Received 30 June 2003/
Accepted 11 September 2003
The
genes encoding the structural components of the nitrate/nitrite
assimilation system of the oceanic cyanobacterium
Synechococcus sp. strain WH 8103 were cloned and
characterized. The genes encoding nitrate reductase (narB) and
nitrite reductase (nirA) are clustered on the chromosome but
are organized in separate transcriptional units. Upstream of
narB is a homologue of nrtP that encodes a
nitrate/nitrite-bispecific permease rather than the components of an
ABC-type nitrate transporter found in freshwater cyanobacteria.
Unusually, neither nirA nor ntcA (encoding a positive
transcription factor of genes subject to nitrogen control) were found
to be tightly regulated by ammonium. Furthermore, transcription of
glnA (encoding glutamine synthetase) is up-regulated in
ammonium-grown cells, highlighting significant differences in nitrogen
control in this cyanobacterium. Nitrogen depletion led to the transient
up-regulation of ntcA, nirA, nrtP,
narB, and glnA in what appears to be an
NtcA-dependent manner. The NtcA-like promoters found upstream of
nirA, nrtP, and narB all differ in sequence
from the canonical NtcA promoter established for other cyanobacteria,
and in the case of nirA, the NtcA-like promoter was functional
only in cells deprived of combined nitrogen. The ecological
implications of these findings are discussed in the context of the
oligotrophic nature of oceanic surface waters in which
Synechococcus spp.
thrive.
* Corresponding
author. Mailing address: School of Biological and Environmental
Sciences, University of Stirling, Stirling FK9 4LA, United Kingdom.
Phone: 44 01786 467784. Fax: 44 01786 464994. E-mail:
mw4{at}stir.ac.uk.
Applied and Environmental Microbiology, December 2003, p. 7009-7018, Vol. 69, No. 12
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.12.7009-7018.2003
Copyright © 2003, American
Society for
Microbiology. All Rights Reserved.
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