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J. Bacteriol., 04 1997, 2678-2689, Vol 179, No. 8
JC Reyes, MI Muro-Pastor and FJ Florencio
In the cyanobacterium Synechocystis sp. strain PCC 6803 we have previously
reported the presence of two different proteins with glutamine synthetase
activity: GSI, encoded by the glnA gene, and GSIII, encoded by the glnN
gene. In this work we show that expression of both the glnA and glnN genes
is subjected to transcriptional regulation in response to changes in
nitrogen availability. Northern blot experiments and transcriptional
fusions demonstrated that the glnA gene is highly transcribed in nitrate-
or ammonium-grown cells and exhibits two- to fourfold-higher expression in
nitrogen-starved cells. In contrast, the glnN gene is highly expressed only
under nitrogen deficiency. Half-lives of both mRNAs, calculated after
addition of rifampin or ammonium to nitrogen-starved cells, were not
significantly different (2.5 or 3.4 min, respectively, for glnA mRNA; 1.9
or 1.4 min, respectively, for glnN mRNA), suggesting that changes in
transcript stability are not involved in the regulation of the expression
of both genes. Deletions of the glnA and glnN upstream regions were used to
delimit the promoter and the regulatory sequences of both genes. Primer
extension analysis showed that structure of the glnA gene promoter
resembles those of the NtcA-regulated promoters. In addition, mobility
shift assays demonstrated that purified, Escherichia coli-expressed
Synechocystis NtcA protein binds to the promoter of the glnA gene. Primer
extension also revealed the existence of a sequence related to the NtcA
binding site upstream from the glnN promoter. However, E. coli- expressed
NtcA failed to bind to this site. These findings suggest that an additional
modification of NtcA or an additional factor is required for the regulation
of glnN gene expression.
Copyright © 1997, American Society for Microbiology
Transcription of glutamine synthetase genes (glnA and glnN) from the cyanobacterium Synechocystis sp. strain PCC 6803 is differently regulated in response to nitrogen availability
Instituto de Bioquimica Vegetal y Fotosintesis, Universidad de Sevilla- CSIC, Seville, Spain.
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