Affiliation:
1. H. Steinitz Marine Biology Laboratory, Interuniversity Institute for Marine Sciences, Eilat 88103, and Department of Microbial and Molecular Ecology, Hebrew University of Jerusalem, Jerusalem, Israel
Abstract
ABSTRACT
Nitrogen nutrition in cyanobacteria is regulated by NtcA, a transcriptional activator that is subject to negative control by ammonium. Using
Synechococcus
sp. strain WH7803 as a model organism, we show that
ntcA
expression was induced when cells were exposed to nitrogen stress but not when they were subjected to phosphorus or iron deprivation. Transcript levels accumulated in cells grown on a variety of inorganic and organic nitrogen sources, with the sole exception of ammonium.
ntcA
transcription was induced when ammonium levels dropped below 1 μM and reached maximal levels within 2 h. Furthermore, the addition of more than 1 μM ammonium led to a rapid decline in
ntcA
mRNA. The negative effect of ammonium was prevented by the addition of
l
-methionine-
d
,
l
-sulfoximine (MSX) and azaserine, inhibitors of ammonium assimilation. Thus, basal
ntcA
transcript levels are indicative of ammonium utilization. Conversely, the highest
ntcA
transcript levels were found in cells lacking a nitrogen source capable of supporting growth. Therefore, maximal
ntcA
expression would indicate nitrogen deprivation. This state of nitrogen deprivation was induced by a 1-h incubation with MSX. The rapid response of
ntcA
gene expression to the addition of ammonium and MSX was used to design a protocol for assessing relative
ntcA
transcript levels in field populations of cyanobacteria, from which their nitrogen status can be inferred.
ntcA
was basally expressed in
Synechococcus
at a nutrient-enriched site at the northern tip of the Gulf of Aqaba, Red Sea. Therefore, these cyanobacteria were not nitrogen stressed, and their nitrogen requirements were met by regenerated nitrogen in the form of ammonium.
Publisher
American Society for Microbiology
Subject
Ecology,Applied Microbiology and Biotechnology,Food Science,Biotechnology
Cited by
105 articles.
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