Nitrate/NitriteAssimilation System of the Marine Picoplanktonic Cyanobacterium Synechococcus sp. Strain WH 8103: Effect of NitrogenSource and Availability on GeneExpression

Abstract

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.

Effects of inorganic nitrogen on taxa‐specific cyanobacterial growth and nifH expression in a subtropical estuary

“…The PCR mix and conditions in the first and second round reactions were identical, except inner nested primers nifH1 (forward) and nifH2 (reverse) (Zehr and Turner 2001) were used in the second round reaction. The narB PCR amplification mix had 5 mL of template DNA, MgCl at 3 mmol L 21 , dNTPs at 200 mmol L 21 , forward (ACNG-G Y C A R C C Y A A Y G C N A T G ) a n d r e v e r s e (CKHCKYTCNSWRTTNGTCAT) narB primers (Bird and Wyman 2003) at 0.25 mmol L 21 , and 2 U Taq polymerase (Promega) with a final volume adjusted to 50 mL with 5-kDa-filtered nuclease-free water. The PCR products were separated on a 1.2% Tris-acetate-EDTA agarose electrophoresis gel, and bands were excised and purified using the Qiagen gel purification kit.…”

Ecological Genomics of Marine Picocyanobacteria

“…Many of the NtcA targets in Synechococcus and Prochlorococcus are genes or operons that encode the acquisition of alternative N sources (276,278), and global expression of these genes is largely consistent with NtcA action (22,147,149,296). Interestingly, the genes encoding assimilation pathways for the main N sources other than ammonium are confined to an approximately 60-kb conserved region of the Synechococcus genome (Fig.…”

Acclimation of unicellular cyanobacteria to macronutrient deficiency: emergence of a complex network of cellular responses

“…The marine strains are adapted to very low ambient combined nitrogen concentrations and do not exhibit the tight repression in response to ammonium as described for the freshwater Synechococcus or Synechocystis strains. Thus, the regulatory role of these proteins in the marine strains remains to be further elucidated and we refer the reader to recent publications concerning marine Synechococcus and Prochlorococcus strains (Lindell et al, 2002;Zubkov et al, 2003;Bird & Wyman, 2003).…”