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Appl. Environ. Microbiol. doi:10.1128/AEM.02606-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Lack of Ammonium Control of Nitrite Assimilation in an Oceanic Picocyanobacterium, Synechococcus sp. strain WH 8103

School of Biological and Environmental Sciences, University of Stirling, Stirling FK9 4LA U. K.

^* To whom correspondence should be addressed. Email: mw4{at}stir.ac.uk.

	Abstract

In cyanobacteria the transcriptional activator NtcA is involved in global nitrogen control and, in the absence of ammonium, regulates the expression of genes involved in the assimilation of alternative nitrogen sources. The oceanic picocyanobacterium, Synechococcus sp. strain WH 8103 harbours a copy of ntcA but in the present study we show that, unlike other marine cyanobacteria that have been investigated, this strain is capable of co-assimilating nitrite when grown in the presence of ammonium. Transcript levels for the genes encoding the nitrate/nitrite bi-specific permease NrtP and nitrate reductase (NarB) were substantially down-regulated by ammonium, whereas the abundance of nitrite reductase (NirA) transcripts was similar in nitrite- and ammonium grown cells. The growth of Synechococcus sp. strain WH 8103 in media containing both ammonium and nitrite resulted in only minor changes in the expression profile in comparison to that of nitrite-grown cells with the exception that the gene encoding the high affinity ammonium transporter Amt1 was down-regulated to the levels seen in ammonium grown cells. Whereas the expression of nrtP, narB, and amt1 appears to be NtcA-dependent in this marine cyanobacterium, transcription and expression of nirA appear not to be. The ability to co-assimilate nitrite and reduced nitrogen sources like ammonium may be an adaptive trait that enables oceanic strains like Synechococcus sp. strain WH 8103 to exploit the low nitrite concentrations found in oceanic surface waters that are not available to its principal and more numerous competitor, Prochlorococcus.